The max no. of prime implicants in the minimized expression with n-variable is 2$^{n-1}$
here we are talking about possible grouping of the prime implicant or no. of variable in prime implicants ??
case teacher give is black k map where i got 4 Prime implicant and if i draw red k map and i increased 1(in green) then also iam getting 4 Prime implicant