rule of thumb is one guide per ft of length plus 1 and the tip-top so a 9ft fly or spinner would likely have 10 guides plus the tip-top. round up for 9.5ft making it 10ft for guide number purposes.
only difference in fresh or salt is size really although some sizes have uses in both fresh & salt. more salt stiks use corrosion resistant components generally but not always.
Terry...... If you will go to the St.Croix website...there is a section that has guide spacing for all their rodblanks..... This would be a good guide for you to start with..... Check it out....