How to know which system I need 32bit or 64bit windows?

It will depend on the type of system you are trying to bulid.By forcasting the software you are going to use and the the type hardware specification you will able to purchase.

Tere are minimum to maximum requirement for the type of operating system. 

• 1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor*

• 1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)

• 16 GB available hard disk space (32-bit) or 20 GB (64-bit)

• DirectX 9 graphics device with WDDM 1.0 or higher driver

It will depend  on the type of system your tryin to build.

By forcasting the software your going to use and what type of  hardware specificatons you will be able to purchase you should  be able to answer your question rather easily.

There are minimum to max specifications for the type of OS.

Ie. https://www.microsoft.com/en-us/windows/windows-10-specifications

it's deppend on your computer if your ram it's up to 2 GO you use 64bit but if you're RAM it's less than 2 so it's recomended to use the 32bit systeme