It means that more than 20 colleges for adults exist. "20 plus" or "50 plus" usually means that the number is close to that, for example, 23.

Thank you so much

Dear Morvarid, "the Pearl" ^_^ it means there are more than 20 adult residential colleges there. here is the definition from LDOCE6 used after a number to mean an amount which is more than that number: examples: an income of $50,000 plus Most children start school when they’re five plus.

Hi i may help u