You are actually looking to get something like this :
You check the current value for the current cell inside your array. 1 means the tile is empty, so you possibly want to spawn an enemy into it.
A cell of value 0 is simply ignored.
At this point, what AllanR meant, is that here, instead of having strictly 0 and 1 as values, you can have even more values (2, 3, etc...) with each value, meaning what "probabilities" of enemy spawn you are going to have.
In that case, you would actually check for this value as subevent of the "For each XY" condition and have several "branches" depending on the formula you are looking to apply.
In that example, with the use of the local variable, I'm simply checking and making it so that if the random value I get for each cell is greater than 7 then I'm spawning an enemy.
That is in this event as well that you can add a counter of enemy if you want to limit their number.
About probabilities and using arrays I had done a tutorial on random drop loot that can be relevant to a certain extent. You might want to check it out.