There is no real one-size-fits-all average number that would work for a party of a given size. It all depends on many, many different factors. Are these magical items important to the plot of the campaign? Does the party work together to easily overcome obstacles without them? Do any individual party members seem overpowered or underpowered? Are you planning on making magic items the primary reward (aside from XP) for completing tasks in your campaign? Are you going to hide them or hand them out openly? Can the party buy or make magic items aside from what you're giving them?
If you're planning on using a ready-made campaign module, I would say go with what the module recommends for a party of that size. If you're building your own campaign, I would suggest this, personally: take a session or two of play to watch how your party handles the stuff they run into. Note if the challenges are leaning towards being too easy or too difficult (with a party of seven it's likely they'll be on the easy side), and the role each party member plays in the overall party. Then, when you want to give out the magic items, give them something that complements their role in the party well, and I would recommend giving them something of a lower strength, like a minor magic item, if the party has had an easy time with your quests, to a more powerful item if it looks like the party needs more help to survive.