The total missiles you can get is 255 that is 51 missile expansions like from the first metroid.
MP1 Wave Buster
After you get the boost ball and spiderball go to where the the ball is found and go to the door above. You need 36 or more missiles to destroy all cracked pillars parts to get to the top and find the Wave Buster. Three missile per pillar part.
Note: If you leave the room the pillars parts that you destroyed will be back.