It was the Charlie Daniels Band, Uneasy Rider, when I first heard the line “Tokin’ on a number and pickin’ on the radio”
After I got understood what it was talking about, it made me better understand some other songs such as **Bob Seger’s **, **Feel Like a Number **, and Steely Dan’s, Rikki Don’t Lose That Number, but outside of musical references, I have never heard anybody call it that outside of song.
If you have, what era and what part of the country or the world.
Sgt Schwartz
Was a common enough term back in the early 70’s, but was only one of many such terms. I grew up in Tennessee and let’s just say I was well versed.
I’m from Oklahoma, and “number” has been a common term for a joint since at least the early 1970s. Maybe earlier. I wasn’t very aware of such slang before the early seventies.
Hard to tell exactly what some writers mean, tho. In these songs, it could be taken differently. Or perhaps even with the ever so subtle dual meaning! 
Yeah. Back then it was a number, a joint, a doobie or a burrito. There was that one time at a concert that an odd-looking fellow came up and asked me “Do you know were I could get a marijuana cigarette?” I, of course, did not.
I don’t feel like this was the intended meaning in the Seger and Steely Dan tunes. I think Seger was talking about losing your identity in the system (work place, Social Security, etc.) and SD I think was referring to a phone number.
Of course, as always, YMMV
Most umabiguously, Neil Young’s “Roll Another Number For The Road” was a crowd sing-along favorite at concerts. I think it’s safe to say they knew what he was singing about!
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Yeah, a joint was often called a number in the 1970s, at least where I spent them, in New York and Colorado.