you could say that a 16' spacing for timber bents will work for "normal" timber sizes(8x8, 8x10) If you go bigger, your engineer will size bigger than "normal" timbers, costing more and possibly being harder to find.
You could probably stick some extra posts in the walls between bents, but if you have summer beams or joists at 20', you'll find them expensive, and heavy to work with....