Not sure if there's a limit, they may prioritise people that are leaving earlier. It maybe worth waiting until 3 weeks before you go incase test and go gets cancelled, then you won't have the hassle of getting a refund from the hotel.
For international travelers who are unvaccinated or not fully vaccinated, they will be required to undergo the mandatory alternative quarantine of 10 days in a designated quarantine hotel in Bangkok, Phuket, or Samui before they can travel to other parts of Thailand.