Ohm ratings on subs give the user a combination of ways to wire your system. The main part depends on what your amp can handle.
If you wanted to run 4 subs, the Ohm configuration will allow you to wire them up so that your amp sees the lowest resistance and will make your amp push a 2 ohm load or even sometimes a 1 ohm load like some amp models can handle.
Below is a Dual 2 Ohm sub using different wiring configurations and multiple subs.
In the last configuration, the 4 subs are wired in series. If you wanted to run 8 subs, you would run two sets of subs in series, then wire the series into parallel to get a 2 ohm load with 8 subs, but your amp at this point must produce at least the RMS power to each sub in the setup as not to underpower them. so in that set up, if each sub handled 250 watts RMS you would need to run a 2,000 - 2500watt amp.
I think you get the idea.