What are you making? For a K2,P2 rib pattern you need at least 4 stitches per repeat. If you are say making a sock cuff, you could cast on for example 48, 56, 64 or 72 stitches, depending on the yarn gauge and foot size. Each number is divisible by 4 so you can do the K2,P2 pattern. Sometimes it says at the beginning of the pattern the range of sizes followed by the numbers of stitches to cast on for each size.

K2+P2=4 stitches

so if you K2+P2 25 times you will have to cast on 100 stitches

because:
4 x 25 = 100

You would have to cast on 100 stitches to get a multiple of 4 stitches for 25 times.

Hope that helps ;0)

im making a sweater and it is in k1 p2 k1 ribbing.
Thank you for your help. it did help