Quality vs Quantity
It's true that less experienced GMs and players will have little idea what makes a good balance, and some guidelines would help.
Along those lines, don't just say X number of magic items, because the quality and function of those items is important.
For example, let's say you've got a Fighter that has a +1 Shield, +1 dagger, +1 Halberd, +1 Longbow, +1 Short Sword, +1 Leather Armor, +1 Arrow, +1 Spear, +1 Club, and a Never Empty Water Flask.
And you've got another Fighter with +2 Chainmail of Immunity to Lightning, +4 Two-handed Sword, and St Donalds Picnic Basket of Holding.
The first one has armor he won't wear because normal chainmail is better, and a buggerload of weapons he won't use. Sure, he knows how to use them, but in combat, he's going to either be using the bow or one of the melee weapons, probably the ones with the better damages. The arrow is an oddball, as it's really a one use item, but some GMs let people re-use them and treat them as permanent magic items even though you only get to make one attack with it in a battle. As to the Flask, who cares? If you aren't in a desert situation it's barely a footnote. At least the shield is good, unless he's using any of the 2 handed weapons.
The second one has a much better loadout, even though he has only 3 items compared to the first ones 10 items. The armor doesn't have a high bonus, but it's special power of lightning immunity is often valuable to metal wearing fighters when facing certain foes. The weapon is pretty good. It's a large damage weapon with a good bonus. The picnic basket is pretty much a footnote as well, but it frees the GM and players from having to worry about keeping track of rations until the GM wants to starve someone, so counting it against an item limit (even a soft one) for the PC is a bit cruel, especially if you understand it's a convenience for the GM as much as for the players. (Really, do you like keeping track of rations? Even the GM has to if you enforce rations because otherwise the players will go for weeks on 3 days of rations just because they don't want to deal with boring micromanagement stuff.)
Ok, now which of those fighters is better equipped? Which one has a lot of useless magical junk.
As a sidenote, I have been in a very magic item poor campaign. By 8th/9th level, a party of 8 characters had 3 magic items. The Ranger had gotten a +1 Longbow, the Paladin finally got a hold of a +1 short sword (just +1), and rogue/mage had secretly acquired a magic dagger from an enemy mage he backstabbed in combat, and later found out it was +4, making it one of the most powerful magic items in the entire campaign. Let's just say when we encountered wraiths and other magic only foes, we ran like crazy because mages have a limited number of spells and die quickly when they get the opponents attention.
