If I wanted my party's spellcaster to enhance/enchant a weapon or piece of armor, is there a limit to the number enhancements it can have on it? I haven't read anywhere that there is a limit, but putting an unlimited number on them seems OP. I also remember the game Neverwinter Nights 2 limited you to 4, but it didn't follow the rules 100%.
Magic Item Compendium updated and clarified weapons’ limits. These rules are in addition to those in the Player’s Handbook: for example, Magic Item Compendium does not repeat, but does not repeal, the rule that non-epic weapons and armor are limited to a +5 enhancement bonus.
But for the purposes of non-enhancement magic properties, we have the following rules:
General Limitation: You have to pay for it
Even self-crafted magical enhancements are expensive. You are unlikely to actually want to have more than +10-equivalent or 200,000 gp worth of properties much before epic levels anyway, simply because it is unwise to sink so much money into a single item. At the very highest levels (say 18th level and upward) you might have considered buying such an expensive item but it is a dubious choice even if it were allowed.
The limit is based on the total enhancement bonus. Each enchantment has, listed as its price, a number, for example Bane has a +1 for its price. A non epic weapon/armor can have only up to a total of +10, and at least +1 of it must be an actual enhancement bonus, and no more than +5 can be actual enhancement bonus.
So for example, a +1 Bane long sword would have the cost of a +2 weapon and has room on it for a total of +8 more on it.
An important note, some armor enchantments have an actual price and not a bonus, these do not count towards the limit.