- Apt. Key and mail.
I’m surprised how many have a mailbox key. (not that there’s anything wrong with that). I have only had to use a key to get mail when I lived in apartments.
I live in a senior mobile park.
Typically one. Just the mailbox key. On days I go in to work I will use a car key for their cars. I don’t go in very often.
99% of the time I enter my house through the garage usung the opener.
Just house keys. Two of them since we have two locks on the front door. Not by my design, it was like that when we bought the house, but my wife likes to lock them both when we go out.
No car keys, we live in a central urban area and now we’re retired we really don’t need to keep a car… just rent one now and again if necessary.
I always used to misplace keys, though. It took me a LONG time to finally train myself into the routine: Keys live in right hand trouser pocket and NOWHERE ELSE, EVER… 
Good spin off question. Where do you carry your keys/fobs? Mine could be in either front pocket. But I have a 50% chance of guessing right (correctly).
Fanny pack. Each set has a specific pocket.
Two- house key and truck key. Our other vehicle has a fob. I find myself wanting to use a key in my wife’s car and wanting to use a push button start in my truck. I suppose when I get my next truck it will be fob only.
Many newer neighborhoods have cluster mailboxes much like the ones in apartments at the end of the block or whatever, rather than individual mailboxes at each house.
ETA: There are individual locking mailboxes that you can purchase for your house if you don’t live in one of those neighborhoods with cluster mailboxes and are concerned about mail theft. I don’t have one, but some of my neighbors do.
0, 1, or 2. 1 and 2 are apartment key and mailbox key. Car is pushbutton. When I used to go into an office it was all badging or spin locks.
Maximum of two
One for the mailbox. I live in a townhouse, and there is a central mailbox location in the middle of the development where everyone’s locked mailbox is located.
One for the front door.
That’s it. For the garage I use the remote or my phone, cars are on fobs. There are backup keys for the cars, but I haven’t used those in a long time.
My keys stay in my purse.
Typically zero. Unless you count the car fob, which stays in my pocket.
#1 - Front door key.
#2 and #3 - TWO mailbox keys. After a small rash of thefts involving levering open banks of mailboxes with a small crowbar, they installed an armored outer box which solved the issue to date.
#4 - Elevator key to garage levels (tiny bit of added security).
Those are the every day ones. Then there are a few padlock keys, a key to the front door of my folk’s place, a pool/gym key and hmmm…that might be it. Finally got rid of my work key 
(which I used to use every day, as every door was locked in the evenings when I worked).
I think you’re misoverestimating the fraction of Dopers who live in detached houses.
Especially as our demographic has aged.
As an example, I moved from a detached SFH to a condo w group mailboxes at age ~55. I’ve moved twice since, but sure never going back to all the expense and hassle of a detached house.
Almost all the physical keys I use are for work: a key to get into the building, a different key for the interior doors, another different key for a locking cabinet in my classroom in which I keep my personal stuff, a key for my desk drawers, another key for my employee locker, and a key to the trash compactor that I use a few times per week.
I have a house key but because I get home after everyone else, the house is always unlocked. My car uses a fob but my wife’s does not so when I drive her car, which is maybe once per month, I use a physical key.
The backyard gate and the garden shed both have combination padlocks.
I carry a key to my dad’s house on my keyring just in case I need to get into his place when he’s not home but I can’t remember the last time I used it.
Two keys (front and back door at home.)
I’m retired and don’t drive…
By the way this reminds me that as a young kid, I assumed the more keys you had the more important you were in society. The building super where I lived in NYC was one of the most important men I knew 
.
Using one or none isn’t too uncommon. More often two; house key and either car or motorcycle key.
One key for house, mailbox, garage, and main door to the multi-family home. The only time I don’t have my key on me is when I am inside.
Badge for work. I can use an app, if I forget my badge.
Car is keyless.
About once every 2 weeks I have to open the safe at work to get a key to open a locked cabinet to retrieve a key for someone else.
In the previous place there was a key for the door and another one for the deadbolt and a key for each of the two cars. So not many more.