Multiple reports in a match don't actually mean more than one. Premades cannot abuse the report system any more than a single person can. Those who repeatedly make false reports have the weight of their reports reduced too. Also, you don't get a notification until you've been reported in a high percentage of your games; 2 games won't do it.
I've had people on my team complain immediately after a game. 'lol which one of you losers reported me' in instances were all four of us agreed the fifth was toxic and we would report post-game. (because they got the popup warning)
I know the system would give you a warning if you were reported often over the span of a decent amount of time, but I also thought that it would give an immediate warning after many reports in a small percentage of games (8 players reporting you total after three games, for example).
Considering that's the only warning I've ever received, I just assumed that Riot thought I was falling off the deep end because of four reports back to back.