Generally, network connections use serial signaling, so bits are serialized onto the fiber. In order to recover the senders's bit and byte clocks on the receiver side, a line code like 8b10b or 64b66b is required.
For Ethernet, some high-speed variants (10+ Gbit/s) use four or more lanes to spread bandwidth across multiple carriers. For short-range multi-mode fiber that usually means multiple fiber strands per direction, and for long-range single-mode fiber it's usually done with a single strand and multiple wavelengths/colors (wavelength-division multiplex WDM).
Also, you can run multiple, independent links over a single fiber pair using WDM as well.
On the other hand, up to 50 Gbit/s can be sent using a single lane or carrier. 100 Gbit/s lanes are currently under development (e.g. 100GBASE-LR).
Additionally, when fiber strands are scarce, there are various bidirectional variants that use a different wavelength for each direction (wavelength-division duplex WDD). You can e.g. use 1000BASE-BX10 for a 1 Gbit/s link over 10 km using just a single strand.