Use System Condition to test how many enemy objects are there.
Compare two values
Compare any two expressions (which can either numbers or text) with each other. They can be compared as Equal, Not equal, Less, Less or equal,Greater or Greater or equal.
Assuming EnemySprite is your enemy's sprite's name, you could refer to the Count variable of the EnemySprite.
For example, with the above system condition, you could put EnemySprite.Count in first expression and the number 100 in the second expression. Then, compare these two expressions as "< less than".
This means, whenever EnemySprite object is less than 100, the condition will be true. You could add your action to spawn more enemies in this condition here.