What kind of felt underlayment should I use?

Felt paper is a vapor barrier plus keeps the oils in the shingles from leaching out and into the wood substrate (i.e. plywood or OSB roof deck), so #15, 20, or 30 will work for any shingle roof. The type I recommend depends on roof slope.

For slopes less than 4/12, I recommend 2 layers of #15. Helps reduce the possibility of leaks. The felt overlaps in "shingle fashion".

Slopes 4/12 to 6/12 I recommend one layer of #15 or #20.

Slopes greater than 6/12 I recommend one layer of #30. The steeper the slope, the easier it is to slip and tear the felt. #30 is heavier so on steeper slopes, you don't slip and tear it as easily.