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