I tend to start with one context per suite
Then break it down when I find I have separate sets of step definitions that don't share state - for me this tends to be emergent
When using multiple suites a good heuristic is to split things common to both suites into one context
Ultimately there isn't a right answer - if you're not sure yet, just split in a way that makes sense and change it later
If your main driver is keeping the files short/comprehendable consider using Traits