that really depends on the application and operating mode. a single channel system of let's say
ssb voice or audio bandwidth data operates within a 3khz slice of spectrum, so the baseband is 0-3khz. within that 3khz slice, voice communication could use up the whole 3khz, but with software that uses the computer sound card as a modem, you could fit 90 or so 31hz data channels. there is some ham radio software that allows one to decode many PSK signals within this 3khz slice. there is also software (which i mentioned as an example above) that uses a whole 1 or 2khz portion, using OFDM to transmit data using 63 redundant channels within that 1 or 2khz space. it's an extremely efficient and robust (it works well even with interference and selective fading) method of sending text or data. this OFDM signal is similar (except for bandwidth and data rate) to the way Wi-Fi works. all of the "magic" is done in the baseband (wi-fi's baseband signal is 20Mhz wide instead of 3khz) and the resulting signal is fed to a modulator, where it is mixed with an RF carrier to create the RF signal that goes out to the antenna. the channel width in the wifi spectrum is 20Mhz, so to switch to another channel, the local source of RF for the modulator is switched to a different frequency. the baseband signal remains the same.