We know that House Redwyne has 200 warships. So Paxtor Redwyne must have around 20000-40000 men to crew these ships, which is way more than what any non Great House can field. Besides that would put House Hightower’s strength at around 100000(thrice as many men etc.) However in WOIAF we learn that the Reach has a post of Lord Admiral. So could it be that Lord Redwyne is simply the current Lord Admiral of the Reach and that his ships are crewed by men from all over the Reach, and not just Arbor?

Well, keep in mind that it’s easier to raise oarsmen than trained warriors. Given that the Redwynes have a huge proprietary trading fleet, I think what happens is that the Redwynes have a “standing” naval force that earns its keep in commerce and providing security, and which then converts over to fully military when the banners are called. 

In terms of the “thrice as many,” I think that refers to fighting men. I estimate that the Hightowers have around 18,000 fighting men (although part of that is coming from their vassal houses), which would mean that House Redwyne has a maximum of 6,000 marines.