How much binary data is transmitted across a physical wire in one clock cycle?
Actually, the answer to your seemingly simple question is more complex than you'd readily believe!
The short answer is that one signal at a time can be passed through a single signal wire, in one cycle. The amount of data that symbol represents depends on the protocol used.
The long answer is that:
2-state protocols, like OOK (On-Off Keying), pass only one bit (on or off) per cycle;
1-dimensional multi-state protocols, like FSK(Frequency Shift Keying), PSK(Phase Shift Keying), FM(Frequency Modulation), or AM(Amplitude Modulation), can transfer a few bits of data at once;
Multi-dimensional multi-state protocols, like QAM(Quadrature Amplitude Modulation), can transmit fairly huge amounts of data in a single cycle (I've seen QAM 512 {9 bits per cycle} advertised).