You are not limited to the number of PIRB’s CoC’s that you can purchase, but rather limited to the number of “unlogged” PIRB CoC, that you may have in your name. This prevents a plumber from just purchasing CoC’s without logging and not allowing for the 5% auditing process to take place. For example, assuming your limit of unlogged PIRB CoC’s is 10, to be able to purchase 5 more you need to issue and log five, which will reduce you unlogged count down to 5 and put you in a position to purchase five more.