in UMTS, each user is allocated a “Code”, rather than a timeslot and a frequency (carrier).
Thanks to this code, the user is able to send its data over the whole band (5MHz by def.), and this data can be retrieved among all the other signals, thanks to this code (each code is orthogonal with each other)
your second question is : how many codes can be handled by each “TRX” in 3G. This, I don’t know. I think it is not standardized and highly depends on the capacity of the HW. It’s vendor dependent.