We know of at least three. Allow me to explain.
In this thread here, where Richard encounters his first PBYR, William makes the comment after his mom messed up his PBYR,
"It was the Best one! Ever!" adds the door..
This implies that there had been at least one built before that.
http://www.fairfaxunderground.com/forum/read/40/2006846/2006870.html
And in this thread here a lurker from this site recounts how he went by the Moreno's home and saw Richard cleaning up a PBYR.
http://www.fairfaxunderground.com/forum/read/40/2190797/2190999.html
That makes at least 3 although I suspect the real number is closer to 20 or 30.