Dual 60 Amp circuit for Well and Tankless Heater
The tankless water heater will have instructions detailing how many breakers of what size. What is the make and model?

The Rheem 36 kW model requires four 40-A double-pole breakers. Probably it has four separate heating elements each 9 kW. I would think you would  need to upgrade your electric service to at least 300 A to supply this heater. How large is your house and how many people in the household?