Depends on the interface. With 18 as an example, this means there are 18 total input channels available for use on that interface. Those channels could be all the same type of input, or a mix of different types. So for an imaginary interface, let's say it has 18 input channels with 6 analog inputs built into the interface. That means as a standalone unit for recording analog stuff, you only have 6 input channels but there might also be an additional 12 digital inputs via ADAT or some other digital protocol. (there are several) In order to use all the channels in that case, you'd need additional hardware to expand the available analog inputs, or you'd be connecting a device that has audio that's already in the digital domain. On other interfaces those 18 channels might all be analog inputs. There are different types. Mic inputs would have a mic preamp built in. Dedicated Line inputs typically would have no mic preamp, so you'd need to use an external preamp if you wanted to connect a microphone. Instrument inputs are similar to mic inputs but have a different impedance value, and would typically be found on a channel that has a mic preamp and both 1/4 inch phone jack input connectors as well as XLR. These are all pro level inputs that would be operating at the +4 dBu line level standard. If you wanted to connect comsumer stuff like home stereo equipment, there is a consumer line level which runs at -10 dBV. Typically RCA connectors, but could also be 1/4 inch or 3.5mm.
The user manual for specific hardware should cover all of this.