Right now, Apple only sells one generation of Apple Watch on its website: the Series 8, SE (2nd generation), and Ultra, which are all from 2022. However, there are far more models than that if you start looking back.
The first Apple Watch launched in 2015, and since then there have been new models every year up to the present. That means an original Apple Watch, a (confusingly named) Apple Watch Series 1 distinct from the original Watch, then Series 2, Series 3, and so on up to the Series 8. The Apple Watch SE, meanwhile, is in its second generation.
There has only been one Apple Watch Ultra generation so far. There also used to be a special high-end model called the Apple Watch Edition, which launched in 2015. However, it was discontinued in 2022, and the last new model was released in 2019.