1000s of sprites, Basic animation

I’m making a game inspired by Dominions 5. As part of it, there will be battles with thousands of units. Each unit has only two sprites, a passive one when they’re just standing or moving around, and an active one for when they’re using some ability. See the following image from Dominions:

The hope is for the game to work on mid-tier mobile devices from years ago. I think I’ve figured out how to handle the gameplay part of things so it’s not too processor intensive, but I don’t know enough about graphics stuff to figure out how to have so many sprites and switch them out. Any advice?