l4d2 can be played in 2 people or must be 4?

You can.. 2 peoples 2 bots

Yes, it can be played in such way. There can be from 1 to 4 players. Other slots will be filled with bots. There are even mods which extend the maximum amount of players from 4 to IDon'tKnowHowMany

I assume you're talking about 2 player coop right? You can have 2 players controlling survivors, Bots will control the other 2 survivors and 4 special zombies.

You can play in any combination you want. (Solo,1,2,3,4) other survivors will be controlled by bots.I've played the game for a couple of hours alone or with in-complete online teammates and i must say the bots are really good and helpful.

2 human and 2 AIs
The AIs aren't really bad
Especially if you're playing in "jungle" environment
They can see (too) clearly and will help you alot

But sometimes they did bad things, yeah since they're AIs and have a lot of bugs
For ex. if there's 2 survivors need help (1 choked by smoker & 1 grabbed by charger)
They'll just stand there sometimes or walk toward the choked survivor and do nothing (just walking around)
But yeah AIs problem

What I mean you can enjoy it alone :)
But not with other gamemodes