How many sweet spots?

It's hard to tell. Of course the K-tab and P-Tab should be 2 points, and I don't know is the anus (external) is a traditional acupressure spot, probably not, but that would count and be No. 3 in my opinion. Then all the internal points... If I'm lying down on a hard surface, then that covers several more points. 

I agree that often, the pressure point won't be obvious because other sensations are stronger. 

The more the better! You will find as you progress that more points come into play and contribute to the overall pleasure! And as they join together in harmony, it becomes even more satisfying!

I have had one or two sessions where the p-tab was absolutely the star of the show. It's so strange.

There are so many including (see glossary) the deer effect. 
Frequently during an aless session I’ll use what I call the four fingered trumpet. Grabbing my right cheek with my right hand, placing pinky finger on the kundalini; ring finger on the anus; middle finger on the prostate area and the index on the forward p spot. Rocking those four fingers in a gentle motion can generate one wave after another, each feeling slightly different from the others. And at some point the body can take over on its own with little effort until I get exhausted. Rest. Repeat. 🤤