There are like 10-15 people on a9 every day in the evening (central euro time), guess, there will be some in about a couple of hours.
2v2 is better because of teamwork. Otherwise you just run/crouch across the whole map for 2 minutes until somebody plants and then one of you gets a kill and win the round.
Since each round takes a min or two and you need 11 points (kills) to win, whole game takes about 20 minutes and you get up to 11 kills in those 20 minutes.
11 kills in 20 minutes = no practice. You already know how to crouch and you can practice quickscoping and stuff like that alone - just run around and shoot road signs, lol.
Teamwork is what most people like SnD the most.
You need to cover and help your teammates and you can't run around one building with your enemy and try to shoot him for a minute because one of his teammates would come and shoot you.