Mac contains 2 memory slots, and each has 4gb , will I be able to increase in to 8Gb each?

That model has 2 slots, each able to use 8gB. You would need to remove the existing sticks of RAM to install newer, higher capacity sticks.

Click the Apple logo in the top left corner, and select “About this Mac”. This will tell you the year of your Mac. Post back with that info.