Only kept sheep for a couple of years so a complete novice.
Think it would depend on many factors, such as
Do you intend to breed, fatten for the freezer,lawnmowers .... purpose for keeping sheep?
Breed you choose. You maybe able to keep more if a smaller breed.
What kind of land you have/ quality of grazing.
Whether you are prepared to give additional feed than the grazing available.
The more kept per acre - probably means more worming needed , more chance of poached ground, disease, etc.
I was given lots of different answers to the how many should I keep/start with. Lots of different opinions out there. Most people I asked said at least 3 sheep.
As I say I have not kept sheep for long so not very qualified to give advice. Hopefully someone will be along soon with more experience.