How many bunches should you leave on a grapevine?

[Olorin2001]

The stem as it enters my greenhouse is as thick as my wrist, there are three laterals running along the roof for a length of 18 feet.
It is just starting into growth and with a bud breaking every three to four inches I could have millions of grapes but they would all be small. Is there any standard formula for how many bunches a vine should be allowed to mature?

[greenishfing]

A few years ago I read you should leave approx. 1 bunch per foot on each rod. That's what I have done for the last few years with good results.