Let's assume I have a DTO with optional parameters, for example:

{
  a: int //mandatory
  b: int //optional
}

What is the best practice, to create 1 test that cover all the DTO or 2 tests, one for all the parameters, and the second with the mandatory only? I am afraid from overhead, in case I have multiple optional, should I cover all the permutation??!! Thanks in advance

I would do two. One with all mandatory only and one with all (If you can). More generally though, cover the scenarios your consumer expects

thanks for your comment, it will not make a lot of tests? or in other words, it verify all the contracts, and this is the aim of pact

The aim is to cover the representative examples your consumers need. You want to be careful not to bother into functional tests though