When more one than one speaker is connected to an individual amplifier, they can be wired in series, parallel or (rarely) in series/parallel.
Speakers in Series
Two or more speakers can be wired in series. That is, one wire is used to to go from each speaker to the next. 
This is not normally used for more than 2 speakers. For a practical discussion on wiring 2 speakers in series, see the article on Connecting 2 speakers to an amplifier.
To calculate the total impedance of speakers in series simply add the impedance of all the speakers together. For example, in the diagram above, if each speaker was 4 ohms, then the total load impedance seen by the amp would be 4 + 4 + 4 + 4 = 16 ohms.
Adding speakers in series is a safe way of adding speakers to an amplifier, but not always practical. The higher total impedance also doesn’t allow the amplifier to produce its maximum power. For more in how impedance changes amplifier power, see the article on How impedance changes amplifier power.
Speakers in Parallel
Two or more speakers can be wired in parallel. That is, two wires are used to connected from one speaker to the next.
This is the easiest and most common way of connecting 2 or more speakers. Wiring each speaker back to the amplifier with separate wires is also wiring them in parallel.
Both of the above diagrams show 4 speakers wired in parallel. If you follow each wire with your finger on the lower diagram you will see it is effectively wired the same as the first diagram.
Due to the total impedance, it is rare you can wire more than 2 speakers in parallel.
If each speaker is the same impedance, it is easy to calculate the total load impedance of speakers in parallel. Simply divide the individual speaker impedance by the number of speakers. For example, in the above diagrams, if each speaker is 8 ohms, then the total load impedance would be 8 divided by 4, which equals 2 ohms.
If each speaker has a different impedance, then it is is easier to use my Speaker in Parallel calculator.
Wiring 2 speakers in parallel is very common in domestic and commercial installs. You just need to ensure the total load impedance of the speakers in parallel is above the minimum impedance required by the amplifier.
If you need to wire more than 2 speakers in a domestic install, you can use a speaker selector switch if the total load impedance is below the amplifier’s requirements. For more details on wiring 4 or more speakers to an amplifier see the articles Connecting 4 speakers to an Amplifier, Speaker Selector Switch summary, and my unique Speaker Selector Switch Simulator.
If you need to wire more than 2 speakers in a commercial install, there are three alternatives. Firstly you can get commercial amplifiers designed to work with a load impedance as low as 2 ohms. Secondly, you can use multiple amps, That is, one dual channel amp for every pair or for every 2 pairs of speakers. Thirdly, you can use a distributed speaker system.
