Few but large for me. Maybe 7 or 8 sheets at most.
I have a event sheet called "Main" and I include all other event sheets here.
I use many groups within each sheet, which seems to work well, as it is easy to quickly scroll and peek in a group without having to deal with tabs (although tabs are extremely useful too, as it remembers your last scroll position in each sheet, making it easy to flick between multiple event sheets) .
Many groups are not so good when you leave all of them open, as this can slow down C3 when there's hundreds or thousands of events visible. It can take 2 or 3 seconds for C3 to react to UI clicks when this happens, but thankfully you can right-click and collapsing all groups and C3 behaves nice and fast again.
With functions, I store most functions next to whatever is relevant to it (e.g. If a function was Init_HUD then I'd keep this near the top of the "HUD" event sheet). Whereas functions that are useful in many places (e.g. Functions that return a value for converting numbers or some other math), then I tend to have these on a Functions event sheet.
I'm interested to know how everyone else structures their events