Spine is unlikely to have a problem until you have more than 40,000 or 50,000 attachments (depending on your hardware), assuming they are not all visible at once. I don't recommend placing so many attachments in Spine though, it would be much better to use conventions that allow you to rig one set of attachments, then create attachments on the fly. Eg, see here:
Asset management with Spine on the Web
At runtime, you are limited to the amount of video memory, which depends on the devices you target. Apps that have a huge number of attachments probably can't load all the images at once. You could package individual images in your app, then identify what images are needed for the attachments in use for the current equipment/enemies/level/area/world/etc, then pack an atlas at runtime containing only those images. This way you only use video memory for the attachments you are going to actually render. The scalability is great but the downside is it is a more complex solution, while just loading an atlas that contains all the images is very easy.
