I like the 6 vs 6 .
Competitively speaking , it allows for so much . You could have 1 , 2 even 3 strategies with your team . There is enough space for diversity between your pokemon and the strategies . Example , i could use 3 slow pokemon and 3 fast pokemon , the slow pokemon i'll use a trick room strategy , while the 3 fast pokemon i'll use when trick room is no longer in effect . And have a strategy for both halves .
Anymore would be too much to take into considerstion , any less would diminish the diversity and strategy making . I consider competiive pokemon on par with chess .