How many times can a Romanian passport holder use a Visa On Arrival at the Malaysia border?

In theory there is no official limit to the number of Visa On Arrivals people from the 17 countries that BUY them when they enter for 2000 baht.

BUT

Some land crossings won't let people who have to buy VOA's get in more than two times in a calendar a year (enforcing the same rule that the visa exempt entry countries follow).

I would say it will work until it doesn't AND you need to check BEFORE you stamp out of thailand if you can get back in or one of these times you're gonna get stuck in Malaysia and have to fly back to get in.

You could also go to Penang or Kota Bharu and BUY a real tourist visa which would stamp you in for 60 days and which you could get a 30 day extension from for 1900baht after you're here