How to ensure learner selects correct number of answers on a pick many before submitting answer?

Hello all, 

I have a quiz where there a few questions that require the learner to select more than one choice on a pick many.  I'd like to either disable the submit button until they choose the required number of choices, or even better have a pop-up (layer) message if they click submit and have not selected the appropriate number of choices.  For example a pick many where they need to select 3 answers from 5 choices.  I originally thought maybe a true/false or number variable with an "if else" conditional trigger but I'm going in circles. Hoping someone has a solution. Thank you.