How many users can easily be acquired in 1 month organically?

It all depends on how users will find you. Once they find you, do they understand you? If they understand you, they signup / use your product.

Users? Well, depends on the product. Example if you will make calculator. There are tons of app for it but people will use which they find attractive and with good features. Not like all everything and make it more complicated. So, as per my pov. It all depends on the product and the hype.