You should probably have one keg ready to be delivered for emergencies, and another keg ready to replace the emergency keg.
That's basically the rule of thumb I've used (or at least tried to use) with good results. One-third of the keg float on tap, one-third filled and ready for delivery, and one-third somewhere else - waiting empty at a retailer, waiting to be cleaned, scheduled for filling, etc. Realistically, you'll never actually have to exchange every keg in the same week, but it gives you a buffer for adding new accounts.
Actually trying to maintain that ratio for multiple brands while simultaneously managing the brewing and packaging schedules and trying not to go bankrupt ordering kegs is the hardest part of brewing, IMO.