Don't know numbers.  I suspect it depends on the amount of training you have
received - the more training the more likely to have a P-Valve.  You can buy
one and not cut a hole in the drysuit.  The valve won't serve any useful
purpose but you can say you have one.

If just saying you own a P-Valve isn't your goal, you will want to consider
the hole in the suit.  It's not really that much work to cut the hole and
ensure the valve is installed without leaking.  Then you will have a working
P-Valve.  This is right where you want to be on the P-Valuable scale.

Years ago I installed a P-Valve.  Still have it installed.  Never used it.
It leaks in (through the tube) when submerged.  It has to be installed to be
useful - the plumbing has to be connected - if you get my drift.  For most
diving that's not going to happen.  That means that most of the time the
valve will serve no useful purpose and it may cause you to get some water in
the suit.

 My LDS sells a lot of DUIs, and installs a lot of p-valves.

If you need a pee valve, then put one in. I don't have one in my
current drysuit because I don't need one for the dives I currently do
in a drysuit. I've had them before in a different drysuit, and I didn't
really like the dive rite or the halcyon. We had a lot of trouble with
a dive rite in my buddy's suit. If you need one, it's worth the
trouble, but if you don't, it isn't.

I've installed my own and a few for others, and it is best done with a
punch. If you're not comfortable doing it, I would send it to Steve
Gamble in Gainesville http://www.gamblescuba.com/