This is just my opinion, not legal advice IANAL
I am relatively certain that 15 is correct, but there is the totally ambiguous "can be readily converted to hold more than 15 rounds" crap and then there is the clause that previously owned and continuously held magazines are exempted, yet I know of no way to prove either requirement if you are forced to do so by LE, and would not advise possessing magazines over 10-12 rounds in Denver