Depends how you mean, "at once" and "in one cycle"... really you have to explain better.
The cycle time has a minimum that increases as you add code... so how much code? In this situation, how are you processing the incoming stream? Are you waiting for some end marker or terminating condition before continuing the loop() execution, or just collecting what you can and moving on?
You see, there are a lot of things missing from this question...
including, why does it matter, and do you have any code example?
Generally, the maximum throughput of a serial stream is determined by its baud rate.