Ha! Great question! That depends on how much you are willing to clean up and how much stuff you want to put away over, over, and over... If you feel overwhelmed at all the activities being taken out and played with while you get stuck putting them away, then take half of them out, roughly. If you are able to handle the amount, then leave it as is.
It also depends on how often you can rotate: the more you rotate activities, the less you can have out; the less you rotate the more you should have out.
You should also consider how it affects you as a mom with a young child to care for. There is no magic Montessori number. Obviously, in an infant Montessori classroom, there are many activities because of the number of children.
I always believe less is more... I don't believe you need to have a room filled to the brim with activities and toys, etc. And as your tot grows you want him/her to be able to put the activities and books, etc., away, so too many would be hard for him/her to manage.
So try ten activities if you rotate them a lot; or 15 if you tend to leave them out and not change them very often.
Can I expect my 17-month-old to sit on a mat for activities?
Nope! Expect your 17-month-old to move all over the place and go from one activity to the next, to the kitchen cupboard, to the bedroom... to basically explore.
You will be the one to put activities away, maybe while your tot is eating or napping (not while they are playing or you'll drive yourself crazy because the minute you put an activity away they will go and play with it again!).
Eventually, you can ask your tot to help you put things away before mealtime, outside time, etc., so that by the age of 2 1/2 to 3 they will be doing it on their own!
Photo by drwhimsy on Foter.com / CC BY-NC-ND