Hello! I think what you're looking for is a variable. Set a global variable like "spawnrate" to 1; this should indicate the # of enemies that spawn when the event is triggered.
When the timer is up to spawn an enemy, repeat the event a number of times equal to the "spawnrate" variable. This way, you can modify the "spawnrate" variable on the fly and the system will always reference it when enemies are created.
So when you want to increase the challenge, have the System>every 60 seconds event increase the "spawnrate" variable by 1.
To create a global variable, just right click on the event sheet and add a global variable. When you want to modify it, use the System action and you should see a section with the global variables listed (set value, add to, subtract from, etc).
As a design tip, I find it easier to create a separate event sheet named "variables" or something like that and store your global variables there. The layouts don't need to have the event sheet set to use, since global variables are accessible at any time.