When our house was built, the contractor didn't install soffits. Each
rafter "bay" (sorry, I don't know the terminology) under the eaves has
3 screened 1" diameter hole. I'm getting ready to build a soffit this
summer, and I'd like to ask for some advice:

My house is 1700 sq ft. with a common attic. My roof has 9 large
dinner plate-sized vents. What I'm wondering is whether I need to
install a soffit vent on every single rafter bay (I know about
continuous vents, but I'd like to stay away from them. I'm an idiot
carpenter, the less cutting the better). Each side of the house has
30 rafter bays under the eaves, this would require installing 60
individual rectangular soffit vents. This sort of seems like overkill
to me and I'm wondering if I can get by with fewer vents than that.

I'd appreciate any thoughts or advice.

"Each side of the house has
30 rafter bays under the eaves, this would require installing 60
individual rectangular soffit vents. (I know about
continuous vents, but I'd like to stay away from them. I'm an idiot
carpenter, the less cutting the better).

Sounds like a good reason to use continuous soffit vents. They only
require two straight cuts the length of the whole run and are easy to
install.