[brian_]

In this video at around time 13:25 they say that new EVs typically consume around 150 to 200 Wh/Km...

Consumption for all production EV (well, passenger cars and light trucks, not heavier trucks or commercial vehicles) are published along with fuel consumption information:
Natural Resources Canada: 2021 Fuel Consumption Guide

I suggest ignoring the silly "electrical equivalent to gasoline" nonsense which is provided in these listings, and continuing to use the energy/distance form, which is given as kWh per 100 kilometres (so 150 to 200 Wh/km is listed as 15 to 20 kWh/100km).

As with any vehicle, consumption depends on test conditions, particularly speed; in an EV, consumption is higher at highway speed than in urban use.

... is that also true for most DIY converted EVs.

Also what could be the typical efficiency for DIY EVs?

I'm interested because I'd like to make my DIY conversion to have a range of at least 50 Km. That would mean that for 200 Wh/Km (worst case scenario) I would need a battery of 10 KW. In a 72 V system, I'd need 6 12V batteries of at least 139 Ah, but I'm not factoring in the efficiency.

DIY EVs are unlikely to be as efficient as production EVs, because their design is not optimized for the electric powertrain. Most people never measure consumption under well-controlled conditions.

Lead-acid batteries make the situation worse, due to their inefficiency resulting from internal resistance, which is most noticeable at high discharge rates.

Rather than thinking of 200 Wh/km as a worst case, my guess is that it is optimistic unless the speed is kept relatively low.