How to make lemon powder using lemon juice like milk powder?
The problem is that you likely cannot afford it. It is done by vacuum, so the machine (rotovap) will cost you about 10 000 dollars, or you can find a few Chinese noname suppliers for maybe 6 000. 

It is an amazing thing to play with in the kitchen, but it will cover the cost of buying supermarket lemons for several lifetimes. 

Any conventional ways of conserving the lemons will result in something entirely different. Certainly not lemon powder which can be rehydrated to something lemon-juice like. But there are other techniques for preserving lemons, which simply create a different end product with different uses. You could ferment them, or make jam, or preserve the peel only, etc. This is the more feasible way to deal with them.