Global variable enemyCount 0
Global variable totalEnemies 20
System -> Every 2 seconds
---- System -> enemyCount less or equal to totalEnemies
-------- System -> Create object yourObjectHere
-------- System-> Add 1 to enemyCount
If you wanted a random delay, change to:
Global variable enemySpawnRate 1
System -> Every enemySpawnRate seconds
---- System -> enemyCount less or equal to totalEnemies
-------- System -> Create object yourObjectHere
-------- System-> Add 1 to enemyCount
-------- System -> Set enemySpawnRate to round (random(1,10))