If it's someone else's boat, don't try to guess what's right; ask them what you can bring on board. If he/she is good with 4 rods, and you're the only one tripping over them, then you're good to go. But really, if you feel like you brought too much, it's more than likely that the other guy feels it worse than you. In my book, a little consideration goes a long way.

That's pretty considerate for an Ol' Dirty Basstard

My boat is small, I only carry at most 8 rods at a time myself. I tell anyone who wants to fish with me to bring 4 or 5 max. Anymore than that makes moving around in the boat a pain.

When I am a non-boater, I always ask the boater how many I can bring, that way there is no problem.

Communication is the key. It never hurts to ask ahead of time especially if you know who you are going to be out with. I had a 14' Lowe and now a 16' Fisher and We do just fine with 6 to 7 rods a piece and two big tackle bags.