How many catalytic converters on this car?

Hello,

This Buick should have two catalytic converters. One of each exhaust manifold.

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

https://www.2carpros.com/articles/catalytic-converter-replacement

I have attached diagrams below for your reference.

Let me know of any questions.
Thank you.

So, could you tell me why car auto part stores are saying it's three. Two in the front and one in the back.

There is a secondary catalytic component that is part of the center exhaust pipe. Most issues happen with the two primary units.

Okay. Thank you. Are the catalytic converters from Ebay that come in a set for the front good converters.

As long as those converters are the OEM style and meet the emission standards they will work just fine.
Keep in mind that some of the aftermarket converters are not as efficient as they need to be. Ideally you want to get something that has a good customer reviews.