“I might see certain people” means you might see certain specific people that you want to avoid (but you don’t necessarily want to avoid other people: only those people).
“I might see some people” is a bit unclear. It could mean the same as above, but then you would use “certain”, so this might be confusing. I could mean you might meet people, and you want to avoid all people, not just specific people. But that is probably not what you meant.