The scale factor for impedance of similar speakers x in series and y units in parallel noted as xSyP =. x/y.
So the ideal way is to arrange polarities and speakers assuming all are equal here; is have an equal number in Series as per number in Parallel.
Rev1 added info
Speaker DCR (ohms) readings are about 50% of the rated impedance or 4 ohms for an 8 ohm speaker.
- Calibrate with leads shorted to use the result as 0 ohms.
- If 8 x 8 ohm speakers were wired in parallel the result would be;
- 4 ohm / 8 = 0.5 Ohms
simulate this circuit – Schematic created using CircuitLab
The speaker impedance is not critical but there are limits usually 4 ~ 16 ohm range. If Z is too high then power is limited by voltage, and if Z is too low then distortion rises with current and you risk blowing protection fuse or Amplifier from excess heat or current.