Theoretically, a 9x9 board = 81 cells, a 2x2 square takes up 4 cells, so the maximum is ~20 squares. But the structure of the pieces themselves is 3 pieces, and they need to be placed according to the 2+2 rule.
Realistic maximum:
• x5 ~x7 -- a very cool game, almost impossible.
• x8+ -- theoretically possible, but you need a perfect combination.
But seriously, in 90% of games, combos only happen x'2, x'3 -- that's already excellent!