At most 5 but usually 3. I use to be the guy that brought 7 or 8 and only used 3 of them.
My tackle bag that I use to jump from boat to boat use to weight maybe 35 pounds. Now its down to 6-7 pounds and I almost never feel like I need something that aint in that bag.
Keep it to a minimum and your buddy will appreciate it. Chances are if your buddy is catching them on something you don't have he will be more than happy to share with you anyway. Catching fish puts people in a sharing mood.