10 is max as far as I know, yes.
The game's really just assuming that you want to play chicken with whether the previous gyms will still be yours by the time you take the next one. If you're out of town you're more likely to actually be able to hold a gym for longer while in towns they change hands at such rapid speed sometimes it's kind of crazy
Most gyms I gotten was 7, but on my way to get into more I already was kicked out of one gym already and decided to stop and got the 6 rewards.
I think its pretty difficult to control 10 gyms because you need to find gyms you can either take over (thats what you usually do) or find one that still have a free spot(rare) or find ones you can quickly level up in order to get another spot(rare).
And then you have to place strong pokemon in there so that you don't lose them again quickly, but you need to make sure that you still have enough strong pokemon to take over other gyms.
In bigger cities is practically impossible to get 10 gyms because there is always someone coming along kicking you out.
In very rural areas its hard to find 10 gyms that you can take over or that still have free spots.
So the sweetspot should in semi-rural areas where you can easily reach enough gyms within 30-60mins but not so much going on that you'll lose gyms again quickly.
Also 3-5AM should be the best time if you want to get a lot of gyms.