If you have 2 speakers wired in series, then simply add their individual impedance together. For example: A 4 ohm speaker in series with a 8 ohms speaker: 4 + 8 = 12 ohms.
If you have 2 or more speakers in parallel and all the speakers are the same impedance, then divide that impedance by the number of speakers. For example: if you have three 8 ohm speakers, then 8 ÷ 3 = 2.67 ohms.
If you have 2 or more speakers in parallel and all the speakers are not the same impedance, then it is easier to use my Speakers in Parallel Calculator.
Note: all these calculations are for connecting manufactured speakers (boxes). They are not used when building your own speaker box and connecting multiple speakers in a cabinet using a crossover circuit. A crossover splits the signal into different frequencies for each of the speakers and makes the total impedance calculation complex (as impedance is frequency dependent). That is why speaker designers get the big money, and as installers we benefit from their expertise.