Don't need a full 5 (but the more the better) but if you can get even 2 other people you can depend on to call out when they see people it makes a massive difference.I know what im supposed to do etc, so like you said, it can be random, so it would be best to play as 5? I wouldn't want to feel like i've been carried though.
As for feeling like your being carried you will learn a lot more from playing higher ranks (I know I do when I play with MG2+). Feel free to add me Hazy87 Im often on for a good few games per week.
I hate it when 2 people on the team buy at silly times and screw up the economy of the team after 3 rounds of the game, it can take ages to get it back.