9 credit hours or 12 credit hours over the summer? (or just 3, 4, or 5)?

I’m planning to take classes over the summer at my local community college to have transferred to my 4 year (nd they all do transfer properly; i checked). I was thinking of taking Calc. II (4 cred) and Phys. I (5 cred) for the summer, but I am also considering adding another course for 3 credits. I don’t know if 12 credits would be too much for the summer however, given that I am also going to be working anywhere between 5-15 hours per week depending on my need, PLUS being involved in undergrad. research (which would be about 8-12 hours per week).

What do you guys think I should do?

Summer sessions are often condensed - you complete a full semesters worth of work in fewer weeks. I’d advise taking fewer credits and doing very well rather than overextending yourself and coming in with poorer grades. Sounds like calculus II and physics would be PLENTY considering your other commitments.