The number of concurrent faxes (in/out) would be limited by the USB bandwidth, the devices doing the sending/receiving and the USB attached storage. I don't know how many you will be able to support concurrently; but, I'd expect more than one. If you try this and find the number too small
I was only thinking of attaching one modem ... this would be for a low volume service.
Looking at the HylaFax functionality, I also worry about the conversion of documents. The processor in this device has no floating point unit so some types of math run very slow. It all depends on the conversion algorithm(s).
I mentioned this on the Hylafax mailing list ... ie. the lack of FPU, but someone said he had got Hylafax running on a much less powerful ARM processor before.