Yes you can make number 1 without winning a slam, its just hard to do so because the 4 slams have approx double the points available than the 9 masters series events. Those events in turn have 2 to 3 times the amount of points available in regular ATP events. In the game you TEND to need about 4000+ points in total to become no 1.
A win in a slam is 1000, Runner up is 700.
IMS - win = 500, RU = 350
Regular = win = 100-250, RU = 75-150
If you are highly ranked then you MUST count all results in the 4 slams, the 9 IMS events, and then your 5 best results from other events.
So you really only need to play 20-24 events per year in total. In general - I try and have a week off before each Slam, in order to make sure that my short term form is close to 100% for the slams, since they are long hard events, especially if you are playing doubles as well.
Hope this helps