I would assume that you can use the amount of powers from magical items that you can physically wear.
However, if you have, let's say, 20 items in a bag of holding, you can't use them all. Otherwise you're just hording abilities. "Whoops! I used my gauntlets of ogre power daily. I'll just take these off, and put on my Gauntlets of Punching. Used that daily. Time to put on my gloves of phasing hands. Now it's time for the gauntlets of kitten petting."
"Hey, pass over those gauntlets of ogre power - I can use that daily!"
"Here, here's the gauntlets of punching, you can use these after that one too.