None, because it's actually great to play single player especially with the new AI updates. But to play with real life buddies, it's a full team of 5.
Not sure why all the negativity in these comments - I think the game is outstanding, especially if you were big into SWAT 4 back in the day. The devs have honestly done a great job and listened to the community feedback. If you watch youtube milsim players they tend to agree the game is great at what it does. There are around 20 maps and each one has so much replayability. They are also insanely detailed and each really tells a story if you look at the notes and drawings. The AI is much improved, to the point where you can have your team complete the mission while you just watch, and the way they move and aim is so much more realistic than bots you see in other games. What sets the game apart from most others in the genre is the tension from avoiding civilian casualties, so you can't just go in blasting, you have to make split second decisions in addition to hitting valid targets. The main thing I wish they would add, which others also point out, is some slider for AI difficulty, because it is at times unrealistic how 'hardened' enemies are - not surrendering, having reflexes of a 12 year old CSGO player, etc.
To the guy who said enemies are bullet sponges - you'd be surprised how many shots it can take to actually drop someone in real life, unless you hit the head, heart or spine, they will often keep going for a good few seconds not even realizing they've been shot. This game isn't like other shooters where it will always take 2 shots to the body to insta-kill; it really depends where exactly you hit them, and whether you're using hollow point rounds, etc.