Well, @Anonymous User , you could always ask them. However, since 5USA only started showing season 17 a few weeks ago then it could be over 4 years until they finish season 20, based on one season per year.
I’m currently watching season 21 on Tuesdays and season 17 on Thursdays.
@Anonymous User I only see season 18, but no doubt 19 & 20 will be added soon.
So, putting on my best “guessing hat” it look like 5USA have first broadcast rights up to season 17. Amazon Prime have 18,19 & 20. And Sky Witness have seasons 21 onwards.