The 70V amp will deliver up to 40W, regardless of the number of speakers. 70V speakers have “tap” settings. These range from as little as 1/4 W (160 speakers!) to 50W+ (No speakers).
For example, for 10 speakers, we'd recommend tapping the speakers at 2W. Most speaker tap settings are like Binary and go from X to 2X then 4X and so on. Likely, you'll see tap settings of .5, 1, 2, 4, 8, 16W. Shooting for 2W each keeps from hitting the max power on the amp – 20W.