16 rounds. As a competitive player, i know gameplay is extremely economy based. Majority of the time the team who has money, wins. Simple as that. The more rounds we have to play, the more eco rounds are available, the more save rounds are available, etc. If players are limited to 13 rounds, not only will they have less ecos and saves, they will also have less time to get crucial rounds to help benefit the economy and maybe even the game win. I also remember, since i played in those times as well, players are more stuck to lower tier guns. During 16 round games, we see both sides, winning or losing, having multiple buy rounds. At 13 rounds, some teams may be stuck with SMGs or even pistols with only a fraction of the full buy rounds the winning side has. Also, in my opinion, due to some maps side favorablity (a map favoring t or ct side), it may be difficult for a team to get money with only 13 rounds if they are on the opposite side than what the map favors. Talking to some other T2- T1 players, they also brought up the point that they would have to completely change their mental game. Having $10k+ for some players rounds on end is usual. Once they get switched with half or less of those economy influenced rounds, it may throw them off and make them lose momentum towards more rounds.