How do players know when to come in and out between plays? It seems like there are always at least a few players coming and and out between every play. How is it done so quickly? Is someone on offense and defense usually designated to count the players, to make sure there aren’t more than 11 on the field?
Well, the coach calls the play, and everyone should know who should be on the field with each play. As far as counting goes, I don’t know if there is a designated guy, but I have seen players on the sidelines counting the men on the field. Sometimes there is confusion, and the team gets penalized.
In high school it was pretty basic. The coach/coordinator yells “Jones! Get in for Smith” and then you run out and yell the name of the player you’re replacing. Everyone who changes in and out often (Wide Receivers, Tight Ends and Running Backs) is pretty aware of the fact that they may be called out after any play. Can’t really speak to the NFL but I know college is basically the same.
I just remembered that on defense the coach/coordinator would yell the basic name of the package and we would replace people that way. All around it looks significantly more chaotic than it really is.
At the pro level and probably all levels, if a coach calls out a certain play that’s designed to be part of a package, the players on the sidelines should know they’re included in that package. For instance, if a certain play is designed to be a nickel package, meaning an extra DB comes in, the DB on the sideline will run in and let the linebacker he’s replacing know to get off (provided that linebacker doesn’t know already). This works well most of the time, but you do occasionally get a too many men in the huddle penalty. There’s also about 20 coaches on the sideline at the pro level, so someone should be able to notice any problems or communicate to the player if there’s a change in the package. (Edit: Didn’t see Frosted Glass’ comment when I posted this, but it looks like we’re in agreement.)
Slight hijack if its ok with the OP. I’ve noticed over the years when there’s a punt return being set, the PR almost always shown seemingly counting the players on the field while waiting for the snap. Any particular reason for this? Is it something that happens on every play and I just happened to notice it only with PRs. Is he counting the punting team’s players in order to set up his return route better? (i.e. there’s two gunners on the left side and only one on the right. I’d better go right.)
Guessing, the punting team is a hodgepodge of players, starters, reserves, defense, and offense. If one of the regulars on the special team gets hurt, his replacement may not know it. And forget to enter the game. Not enough men on the field
Or there might be two replacements for the injured player and the team might have too many guys on the field.
By the time these players get to the NFL, I would guess the substitution pattern is 2nd nature.
You can usually spot punt returners counting the number of players on the field. Occasionally safeties, too.
Perhaps he’s checking to see if all the players on the punting team are in their usual spots; if someone’s in an odd position, it might indicate a fake punt attempt.
Back to the original question - to me, it always looks like the sideline is chaotic, I guess that all the offensive players must hang around in one area, and the defensive players in another. That way they’re always near the coach calling out the plays/packages.
Unless you’re the Saskatchewan Roughriders playing in the Grey Cup. 
Twelve men on the field just looks like too many if you watch a lot of football. It’s probably very apparent to players having just run the plays over a million times and cases of “Wait a minute, WTF are you doing? I’M the Mac Linebacker?”
Also, a good number of 12-men penalties (technically “Illegal Participation”) happens not when there’s too many on the field ready to play, but when a sub is not able to get of the field before the ball is snapped.
Sometimes nobody notices: The 1969 Orange Bowl
Beat me to it, but I’d have swapped out the smilie for this one 
Had the Roughriders stopped at 11, they would have been fine, since you can’t be called for having too few men on the defense (too few on the offense is an illegal formation). The sad thing is they supposedly had 2 people designated to count the men on the field, and neither person caught the 13th man!
Let’s hope they repeat their error again this weekend!
Is too few on offense an illegal formation, or just too few on the line of scrimmage in Canadian football?
I really don’t know, sorry.
I don’t believe that there is a penalty for too few players on the field, if only 10 play a down. I think that’s more of a “too bad for you.”
The offense does have rules in regards to how many need to be on the line and how many are off the line. Penalties have been called if linemen are a step back from the line of scrimmage and if slot recievers are too far up.
Specifically (in American football):
- The offense must have exactly 7 men on the line of scrimmage.
- The two “ends” (i.e., the man on each end of the 7 on the line of scrimmage) must be eligible receivers (i.e., their jersey numbers must be among those designated for eligible receivers, or they must have checked in with the referee as an eligible receiver before the play).
Isn’t it at least 7 men on the line? It used to be.
I was at a game and I figured a fake punt was coming up because the team went into a huddle before the punt. Normally they would just line up without a huddle. They did fake it and picked up a first down. I don’t think the other team noticed the change so the fake caught them by surprise.
Hmmm…I thought it was “exactly 7”, but apparently I was wrong. From the NFL Rules Digest:
*Offensive team must have at least seven players on line.
Offensive players, not on line, must be at least one yard back at snap. *
I suppose that, in a goal-line formation, when it’s obvious that you’re going to run the ball, you might go with more than 7 on the line. As only the two “ends” are eligible receivers, if you have more than 7 on the line, you have fewer eligible receivers, so such a formation would have fairly limited usefulness.
Additionally, when a starter wants off the field, the usual sign is tapping oneself on the top of the helmet then going to the sideline. His backup will know to go in.