Every sprite has a .count expression. Assuming you have one sprite for your enemy, and that enemy sprite is named Enemy, then the expression to find out how many enemy sprites you currently have in your game is:
If you use the System event: Compare two values you can compare the Enemy.Count expression with however many sprites you want to limit in your game to see if you are under that amount.
The screen shot I posted is exactly what you would have in Construct 2 to figure this out.
If you have more than 1 enemy sprite you're wanting to check on you've got two options:
Place all your enemy sprites into a family and then use that family's .Count expression.
Or you can add all the .Counts from each sprite (Enemy1.Count + Enemy2.Count + Enemy3.Count) in the compare value event.
The family method is better IMO as families make your life better in general.
For more info on common expressions see this manual entry