Per IGN - PokeStops , there does not appear to be an overall maximum.
There is a maximum number Pokeballs you will receive, as well as randomly receiving various other items as well.
(Emphasis my own) Most often, a PokeStop will grant you three to four items at a time.
Until you're level 5, PokeStops will grant you exclusively PokeBalls and Pokemon Eggs.
Once you reach level 5, at times PokeStops will generate up to six PokeBalls, and other items such as Revives and Potions to use now that your trainer can access Gyms and battle other Pokemon.
gaming.stackexchange.com/users/124566/Wondercricket
Just today I received 9 items for the first time from a pokestop.
I have received 8 items several times.
Some time ago (don't remember the site)
I have read that there isn't actually a random in receiving a -x- number of items.
Every 10th pokestop you will receive the bonus of 6 or more items.
Only rule is that it have to be 10 different stops (so without taking a stop for 2nd time before receiving the bonus)I have used this info alot and its true and helps alot.
gaming.stackexchange.com/users/164144/Peter
The other day I had just woken up
I got 21
gaming.stackexchange.com/users/171895/VasectomyLOL
The number of items you receive from a PokeStop is random.
You can receive from 3 to 9 items from a PokeStop, but the most common amount is 3-5.
There is also an XP bonus if you receive more than 6 items from a PokeStop.
If you get 3-6 items, you receive the regular 50 XP.
If you get 7-9 items from a PokeStop, you will get 100 XP.
Update:
content above the horizontal line is based on information from the last time I played, in late August.
With the new app updates, it is possible to get numerous items, up to 15 and more!
gaming.stackexchange.com/users/122845/Vincent T
The number of items you can get from a PokéStop at once (not counting the streak bonus) is 3–9: if you are in 1–6 day streak, double the items, if the seventh day of the streak; assuming that 9–15 items or even greater will drop.
gaming.stackexchange.com/users/172595/333-blue
While walking around yesterday, I hit a quite a few of Pokestops. A majority of the time, I received anywhere between 2 - 5 items from the Pokestop. After a few hours, I assumed that 5 was the cap. However there was an instance where a received 8 items from the Pokestop.  What is the maximum number of items I can receive from a Pokestop?
What is the maximum number of items I can receive from a Pokestop?
gaming.stackexchange.com
<pokemon-go>
gaming.stackexchange.com/questions/273723
gaming.stackexchange.com/users/124566/Wondercricket
There does not appear to be an overall maximum. The number of items you receive is random. It doesn't have an overall maximum and is random.
Users suggest it is random