Ardour can run VST plugins, so I don't think it's limited in this way at all, as VST is the most common plugin format in the world and there are literally thousands of VST plugins that one can use.

Ardour can also run LV2 plugins, which is another plugin format. It's not as popular as the VST format but gives Ardour users even more options.

So, yeah, I wouldn't say that plugin availability is a limiting factor for Ardour, any more than it is for any other DAW.