how many GB in Intel Iris Pro Graphics 6200

The GPU in your 2012, 21 inch screen iMac is a integrated GPU, NOT a discreet GPU, meaning that your integrated GPU uses a variable chunk of the physical installed RAM in your iMac on a random basis, instead of having a GPU with discreet, independent RAM ( seperate RAM that doesn't rely on a computer's physical, installed RAM modules ).

The more graphics power that is needed for a task, the more physical installed RAM your GPU will use.

So, if you only have 8 GBs of RAM installed in your Mac, the OS will have to share physical RAM with your GPU meaning that your iMac model is not always using the full 8 GBs of RAM for OS X, but is sharing a variable amount of RAM with the GPU which can mean slower performance either from OS X and your iMac's CPU or slower graphics response times from the GPU if OS X needs more RAM.

With a computer with an integrated GPU the more physical RAM installed into the computer the better!

You can't install more RAM into any of the 2012 and later 21 inch screen models as these models are, basically, glued up, sealed up computing "appliances", now!

Also, the 5400 RPM hard drive in that 21 inch screen iMac is making your disk read/ write speeds 33% slower, also.

So, to answer your question the VideoRAM (VRAM) in that iMac's GPU is variable based on the amountof available physical RAM installed.