As many users as all of nostr. I can't name all clients that have integrated marketplace, but how nostr works is that these clients are all integrating the same protocol. So any post made on shopstr.store for instance, are the same marketplace posts that show up on any other nostr client with marketplace integration.

I know Amethyst, shopstr, plebianmarket, coracle have integrated this. I'm sure there are a ton more. For some reason the marketplace drop-down on nostrapps.com is broken, it check there too. I let @Karnage know it was broken already so I'm sure he'll fix it soon.

Here is the NIP: https://github.com/nostr-protocol/nips/blob/master/15.md