12 volts if your running four.
However wottys cant run on 12 volts and I think the min of a wotty EV is 14volts. I am not aware of any speedo which can run on 12volts.

There are those tiny speedos on ebay which can aparently handle 200 at 12volts. However didnt want to waste my money and try this as the wire was so thin that I did not beleve it.

One idea that I was playing with is run it on 24 volts and wire up the motors in series giving each motor half of the volts. However im not an electrical expert and Im not sure if thats recomended or not.

Dont run them at 24 volts. They would pull around 1.2 kw and I am not sure if a wotty 360 can handle that. Several people have blown there speedos running them at these voltages.

A safe voltage I would try is 18 volts which on paper should draw the same current as a bosch 750.

I was trying to use 4 of these in a low cost heavy weight. However it all seemed too difficult to implement and it was a lot simpler to just invest in some bosch 750s.

I think white knight uses bosch 400s be interesting how they run theres.