Question: What are the chances of the Strawhats evolving into an armada like those of WB pirates?
Highly unlikely. The Strawhats' objective in mind is very different than those of other pirate crews.
They (Strawhats) yearn for the freedom to go where they want to go, do what they want to do (within their moral values), to sail the world wide seas in searching for adventure and from what it seems, obtaining One Piece will symbolically mark a milestone achievement along the way. If they want, after obtaining freedom (One Piece), they may take another trip into Grand Line, exploring different routes (during Paradise) and different locations (during New World). See different things, eat different foods, meet different people and culture, explore different geographical location, etc etc, so many things to do while one is young and freedom is in hand.
Many of the others have the mentality of "I have come to NW and I'm planning to stay and make a name for myself". They desire to stay put, make an influence and expand that influence for as wide as they possibly can. WB pirates, Kaido, Big Mam, Shanks as well as the new-Yonko-on-the-block, BB pirates, all exhibited such characteristics. Because of this kind of objective in mind, thus the need to grow the crew into a large enough structure capable of defending one's turf and/or expand their influence further into unclaimed or claimed territory.
One thing about territorial claims, it weights down on the owner. For the backpacking travelling adventurers, the Strawhats, they should not associate themselves with any of the locations in the world, otherwise it may severely hinder their freedom to travel freely. Remember Strong World? What was their reaction when they heard about Shiki's plan to harm to East Blue? They get prepared to quit their adventure and immediately return to East Blue. The point is NOT about East Blue being claimed by Strawhats but instead the idea of having bound to defend locations (associated under their jolly roger) under threat while the protector is always "away".
Summary, a large armada (and also territorial assets) will severely limitting the mobility of the Strawhats in their quest for adventures. Therefore, it is unlikely for the Strawhats to grow into a large structure crew like what has been recently revealed about the BB pirates.
So, what is your say?