Under Thailand's Condominium Act, foreigners can own condo units outright, but only up to a limit of 49% of the total saleable floor area in any single building. The remaining 51% or more of the building's units must be held by Thai nationals or Thai-majority entities. This quota applies per building, not per development or per company, so two towers in the same complex can have different foreign ownership levels.
In practice, this means that before you make an offer on a unit, it's worth checking how much of the building's foreign quota is still available. Popular buildings with strong foreign demand, particularly smaller and more affordable units in beachfront locations, can fill their quota faster than larger or less central buildings.
If a building's foreign quota is already full, you can typically still buy a unit there, but not as outright foreign freehold ownership -- instead it would usually be structured as a long-term lease, or purchased under the Thai quota through a different ownership arrangement. Both of these are different transactions with different protections compared to straightforward freehold ownership, so it's worth knowing which situation applies before you commit to a specific unit.
The building's developer, the juristic office (the on-site management team), or your agent should be able to tell you the current foreign ownership percentage on request. It's a simple question to ask early, and it can save you from getting attached to a unit that turns out to need a different purchase structure than you expected.