The important thing here is that we include 'ADD 1 TO S' so that every sprite that gets initialised is added to the Sprite Counter (S) - we use this throughout Shmupkit to ensure we keep track of how many Sprites are currently on Screen, since MPAGD has a maximum of 12 sprites on screen at any one time, HOWEVER, I don't recommend aiming for 12 as I find a general slowdown and the potential for things to go wrong, aim for 10 or 11 and you should be ok.
Remember though that Your Player, Player Missile & Enemy missile will count as three sprites, so in reality you can probably only get 8 alien sprites on screen at any one time.
EVENT INITSPRITE ADD 1 TO S ; increment the sprite counter IF TYPE 1 ; PLAYER TORPEDO LET SETTINGB = 0 ; initialise the distance counter ENDIF IF TYPE 5 ; ENEMIES LET SETTINGA = 0 ; Initialise SettingA ENDIF IF TYPE 7 ; SPAWNER LET SETTINGA = 0 ; RESET SETTINGA ENDIF