Generally around 11 rings including the tip on a 12' barbel rod.
Sometimes the customer will insist on a certain number of rings and size.
Spacings will depend on action.
Some rings (double foot) flex more than others.
Fitting of too small rings at the tip could lead to more weed clogging, but generally give less weight to the tip.
The weight of rings sometimes can be a factor - more so on lighter rods
There are other factors.
Other consideration - rings are time and money to fit.
Fit enough rings to give a smooth line pattern that follows the arc of the rod under tension without any sharp angles. Test the layout by temporary fixing the rings and then threading them up - a nice bright length of fly backing can improve visibilty during tests.Try turning the rod upside down and seeing if any line touches the blank down. Pay attention to the position of the butt ring, reel type normally used.....etc etc.
The above may have you adding or subtracting a ring or two.
So much more but as rule of thumb a ring per foot or maybe one less.
Double rings less flexible than singles but far more robust. Use 'decent rings' - lot of bull regarding hardness/braided lines.
Look at carp rods with 5 rings or less to get completely confused with certain theories.
Alternatively, just build the rod as you like it and go fishing.
Count the rings/guides 9' #6 Harrison Lorhic fly, float rods have gererally more and on and on.........
Tired now, need to rest.
Cheers
Bob