In big fleet battles, sprites/particles get to around 3000. It's mostly due to ship, missile & weapons trail effects, which is made up of lots of fading sprites.
Performance is not sprite limited, it's CPU logic limited to process my AI & Pilots.
I test it over wi-fi on the browser of devices and look at usage statistics and base my optimizations on event groups which are using too much CPU.
The only issues I've had are with audio but there are work-arounds for those.
It is the GPU that I worry about most.
Okay... yea. We have between 10,000 and 100,000 star sprites, then fleet sprites, movement path sprites, fog of war sprites, and sprite fonts. So, I think we will need to build a lighter mobile client later. :-p
Your game looks like an RPG version of Distant Worlds, which I like a lot.