Indeed, there's player limit in official servers.
Checkpoint (Co-op) is 8 players
Push (PVP) is 28 players
Skirmish (PVP) is 32 players
Firefight (PVP) is 16 players
Competitive firefight is 10 players
Team Death Match is 24 players
However, you can modify this limit on community servers. Once the XP is implemented, you'll be able to get unlocks as well.
We designed the mode to be playable with 8 players, essentially. This has been something we had in the original Insurgency too. We tested at various player amounts but found that 8 players fit best for the amount of enemy bots we wanted, the frequency of counter-attacks, the frequency “second chances” when almost all of your team dies, etc. Just the overall pace of the mode and gameplay we were going for we found worked best with 8.
That’s for Coop, Checkpoint specifically. And it attends to our reasoning behind the other modes too. Basically, we set player limits to what we feel plays best for that particular mode on our maps and our target goal in terms of pace. Secondarily, we also take optimization into account.