Serious question, may be a silly topic, i apologize

I was wondering how do you pluralize the word "penis". Is it "peni"? Is it "penises's"?

Since some background for this will help, I was in the locker room recently and there was many a naked man walking around and I was texting my gay friend about how many penises's (?) I was seeing and he corrected me to say "peni", which I said didn't look right, but I thought he's gay so I guess he would know.

For the record, I'm not gay. I just sometimes touch guys penises until white stuff shoots out.