It will be helpful to read about "Horizontal microprogramming", "Vertical microprogramming" , "Hybrid microprogramming" before reading answer.
This Question is somewhat wrong.
It must say that out of 25 control signal particular 2 control signal can be active at same time . Then this 2 must be Horizontal, remaining 23 in vertical. for 2 bits we need 2 bits in contro signal , for 23 bits we can use decoder log2(23) = 5 bits ,
Hence total 7 bits are reuired.