I've run into the unit limit a few times so far, and every time I'm surprised why I can suddenly not produce units or build buildings anymore.
My suggestion is to give the player some kind of signal that they're approaching the unit limit, for example by displaying the current unit number and the unit cap when they reach 90% of the unit cap.
Another idea could be a voice line saying something like "you're 90% to the unit cap. Once you reach it you will no longer be able to build units or buildings"
Suggestions/Ideas?