How do I know how much Ram do I need with my this CPU: Intel Pentium G3260 Processor (3.3 GHz, 3 MB, LGA1150 Socket).?

Yeah, only a few games ask for more than 8gb of RAM, but that's with high end systems with high end graphics cards. The graphics card is the determining factor with how much RAM is being used.

I would run a 64-bit os, get at least 8gb of RAM.

If you want to game at higher settings then look to buy a Core i5-4670, a GTX 1060 or RX 580, and a new power supply.

You can still game at maximum settings with that setup if you look for a Core i7-4790, GTX 1070 or higher and a new power supply.

8 is fine for gaming etc

Most programs that people use are still 32 bit. That means they can't use more than 4 GB of RAM. Windows 10 uses about 2 GB so 8 GB is plenty for most games.

The newest AAA games are starting to ask for 16 GB of RAM though, but with that slower CPU you won't be playing those games, so I'd go with 8 GB.

8 is fine for gaming etc

Pentium G3260 supports maximum 32 GB of RAM DDR3-1333.

It depends what you want to do. For general home/office use 4GB RAM probably adequate. For better performance such as gaming 8GB. For specialist applications such as video editing creation, CAD. photoediting 16GB or more