How to make MUI?

Have an ability "Astroprojection" which is an aura affecting enemy units.

Level 1: 25% chance when an affected unit casts a spell that 1 astroprojection will be created; Max of 4 projections
Level 2: 50%; Max 6
Level 3: 75%; Max 8
Level 4: 100%; Max 10

The number of astroprojections is used to determine the effectiveness and damage of other spells this hero has, and although I havn't crafted the triggers for it yet, I see it as possible to do it without worrying about MUI, but I can't even fathom how to do it without the use of globals and whatnot.

Any idea on how to code this so its multi instanceable?

Do you care about using up custom value? If you don't, you could make a code with locals, and then store the number of astroprojections a unit has as it's custom value.

What's your code for it?

EDIT: Alternatively, you could use Handle variables through some workaround (though I'm not sure exactly how; integers aren't handles) or Game Cache.