Actually, I should add that even in the unlikely extreme scenario, the upper bound would be the number of candidates on the parties' lists (Landeslisten). Even if a party would have a right to more seats, they cannot fill more than they have approved candidates on their lists. One could determine this actual upper bound by looking at each state's lists, which are public of course, but I haven't quickly found them compiled in one place, so I won't bother.

(Berlin's pirate party once won a surprisingly high share in the state parliament but wasn't able to fill all seats, because they were polling way lower when compiling their list.)