To late to edit, but I should correct my terminology.
Coil impedance (ohms) divided by number of coils = final load on a parallel connections. Example 2 dual 2 ohm subs wired parallel = .5 ohm load and so on.
A series connection you simply add up the total of the coils to be wired in series.
Example 3 single 4ohm subs wire in series = 12ohms, but not that anybody would do that.