Natural crabmeat is added only for a marketing purpose.
Any kind of natural crabmeat can be used. As far as the maximum level is concerned, it depends on the quality of surimi proteins. The high quality proteins can hold lots of nonfunctional ingredients including crabmeat mince. If the large quantity is used, it will reduce texture and color quality of surimi seafood. For your final question, there is no difference in taste whether the natural crabmeat mince is added or not.