Brian is probably the best person to answer this being an avid fly fisherman in real life,
Depending on which context you mean most fisheries,Clubs or Associations have a set Limit on how many
of each specie you are allowed to keep,if it is a Private fishing pond the owner deems how many fish you can take.
This would cover the second part of your question,if you are fishing a River which includes these and other species,
the Rules for that particular venue should be known by you before you put fly to water,fines are heavy for illegal Fishing.
I hope this helps in some way.
I am so glad to be part of this