How many border crossings allowed at Mae sai border at this time?

Im about to go for the third one and so far nobody has said anything or written any numbers in my passport but I keep hearing rumors of

three or four crossing limit..

There is no official limit, but 4 back to back entries at land border crossing may lead to questioning by immigration as to what you are doing.

If you get stamp in blue=no problem.

Red entry stamp=problem.

I would not read anything into the different colour stamps used by immigration. Many reports in the forum of people having blue, black, purple, red stamps. If you are allowed entry into the Kingdom you will get an entry stamp; the colour does not matter. The only 'red stamp' to worry about is that sometimes put in the passports by consulates when issuing tourist visas, indicating this may be the last time of issue.

There was a report from a Russian citizen at the the Cambodian border that was told "last visa exempt stamp for you" and got a red stamp.

Russians gets 30 days at land borders from a bilateral agreement

I would not link one report (where full details have probably not been provided and there may well be other circumstances leading to immigration's decision - if indeed the report was correct) and a red entry stamp to equalling some change in immigration policy/procedure. I for one have a red entry stamp in my passport from a few months back, and have since re-entered the Kingdom without problem. So, in isolation, it would appear to mean nothing at all.