How rare are magic items in D&D5e? How often do a bandit drop a +1 sword or how often can the party find a bag of holding? After reading though the DMG, I cannot find anything about how many items a party of four should have at lvX and so on.
I have previously played (as a player, not DM) D&D4e, where Magic items feels a lot more common.
5e was intentionally designed to rein in the amount of magical equipment assumed in earlier editions.[citation needed] By design the game can be played 1-20 without a party ever seeing a magical item.
On the other hand, you're free to hand out +3 armor to your level 1 party. So how do you choose?
First, the DMG does have some (oblique) guidance on this: notably the section "Starting at Higher Levels" and the table "Starting Equipment" on p.38. While it's not exactly saying "this is how much you should have," it gives you a feel for what the designers had in mind when they were thinking about magic item prevalence.
Second, look to WotC-published adventures. Grab a few Adventurer's League modules from DM's Guild, or take a look at one of the hardcover adventure campaigns. Again, not that you should give that much in magical equipment, but it gives you a feel for what's happening in a lot of places. (And here's a bit from the lead designers on how and why they choose to distribute magic items how they do.)
Third, if you're still feeling unsure then lean toward consumable magic items rather than permanent ones. That way if you come to regret dishing it out, at least you won't have to live with it for long. Compare a +2 shortsword and the shortsword of Maximus Virilius (which trapped the essence of the kings he slew when conquering Whereizzit; it currently contains seven charges, each of which gives the sword +2 for a day). Same mechanical effect, but one (a) adds interesting decisions and (b) will, eventually, not be a factor.
