what is the Max limit of creating rule inputs in an expression or in a Interface?

Without a review of the interfaces it's hard to say if they're poorly designed (i.e. perhaps they should be decomposed into smaller objects with less rule inputs each). The effect is likely to be around understanding who they work and the effort required to full test them. There are no real run-time effects as a rule input is simply a "pointer" to a piece of memory that holds a value and whether you have one interface with 20 rules inputs, or 4 interfaces with 5 rule inputs each the run-time effect is the same.