A. You are only limited by how large the registry grows. I know of sites with 990 print queues, all lpr, on one NT server with no performance problems - NTS 4.0/SP3 dual P200 Pro, 256MB, FDDI.
There are some variables, but each queue adds about 30-35 KB to the registry. By periodically eliminating the wasted space and removing LastKnownGood as above, the limit is probably somewhere around 3500. I would expect performance to be a problem before this limit is hit.