Assuming they operate the same way Mastodon itself handles federation and only relay the content people follow, there's no reason you can't fairly straightforwardly scale it by just running more of them. But frankly I'm not convinced Bluesky has much of a future. It needs to 1) offer something significant over the wider Fediverse - which is far more than "just" a Twitter-like and they only have a partial answer to the Twitter-like part - and 2) prevent being embraced and extended. E.g. nothing but inertia prevents the same level of indirection to provide smooth migration as Bluesky offers, and so if users care it will come to other services over ActivityPub too. The good parts of Bluesky will be copied. Then what? Especially if you're on the Bluesky side of a bunch of bridges where people on a lot of the instance on the other side look at you with suspicion and lock you out from their communities.

The limitations on filtering on Bluesky and the culture of filtering on Mastodon where this ability is seen as a community good means that if you want a 'global' view of the combined network, you're going to need to be on the Fediverse side of those bridges. That's not a good position to be in for Bluesky.