If I had to guess, I would say at least 15,000 if not the full 20,000.
What is, unfortunately, not surprising is that the English language Thai media completely neglected to report on what was the rational for the protest.
While it is easier to draw the palace and pretend it is an affront to the King's over-compensating birthday celebrations that is not the case.
It is Constitution Day, and considering paramount issue of the military tearing up the 1997 constitution, there is a serious reason to protest on this day.
But, as the largely partisan Thai press is prone to do, they neglect reporting on the issue at hand and churn out either useless or biased stories that fail to inform readers what the issues at hand are and what BOTH sides are saying. God forbid they let the reader decide who is right are wrong.