Yes it is correct to say either "real estates" or "real estate". Both terms are globally used interchangeably, depending on geographical origin or intention for vocabulary use.
The application of singular and plural does not really apply when using the term: Real estate. Because real estate is already a plural term defining a combination of lands and building that is owned. Therefore the term "real estates" would be a term defining the various modes of lands and buildings owned.
You could thing of it in a similar manner as the term "fish" and "fishes:. Where as fish refers to one or more than one fish. Fishes refers to more than one type of fish.