How many sprites is too many?

I realise this is kind of a "how long is a piece of string" question, but what's a good upper limit to have for the number of sprites in a layout?

As a bit of background on this, I'm working on a tile-based mining game. At first I was generating every tile sprite individually, which meant I was starting every layout with around 7,500 16x16 sprites from the get-go. I've since switched to using the TileMap object and the framerate gain has been phenomenal, but it's got me thinking about more ways to improve efficiency.

For instance, I have a black "darkness" layer above my main layer, and I was generating a Destinationout "lightwindow" sprite every time the player mines a tile, so that only the area that's been mined out is illuminated. These lightwindow sprites mount up after a while and start impacting framerate, so I started using Rojohound's very excellent Canvas plugin, pasting the lightwindow to the canvas and destroying it on creation. This works nicely, but a one problem is that pasting to canvas causes a very slight stutter, so the compromise I'm considering is to let the lightwindow sprites mount up until they start having an effect on framerate, then pasting & destroying them all at once - this way the stutter only occurs occasionally.

Anyhoo, with this in mind, how many sprites is too many? I realise all systems are different and games will always run less well on less capable machines, but are there any good rules of thumb for this?

Id say that there's really only one rule. Testing, Testing and more Testing.

Have a targeted device or SoC performance level you want your game to run well on first and foremost. Then work based on that. At some point there has to be a cut-off where devices with weaker SoCs are just not going to handle it well, its up to you what you want the cut-off point to be.

Guess I'd better get testing then, thanks for replying!