I haven't seen the latest Thai reports. The media usually refer to the most recent WHO report from 2015, based on data from 2012/2013. They have a methodology of collecting data from different sources: for example Thai Highways and Thai Health Service have different totals. Then they adjust for how each country counts, or doesn't count, victims. Then they add a subjective adjustment based on reputed expert assessments. Probably it is the best SWAG available. That last report gave the Thai death count as 24,237 persons.
Data are here:
WHO | Country Profiles 2015
Whatever the exact number, it is a shocking waste of human life.