Sacrificial anode calculation method?

You mean without the number of anodes? So let's figure out the surface area S that we need to protect the pipe or the tank.

The number is the number of anodes needed for construction

That means you're multiplying the number of anodes by 1.1 or 1.2. The number of anodes is obtained by dividing the total current density by the output current density of the anodes. But make sure that you don't get underprotection so add a 10-20% margin. According to the surface current density I of the protected sufficient, the total current density I required for the protected sufficient is total =S x I.