How much power should I run to my workshop?
power a small heat/ac window unit [ How much current will that use ?  I am thinking at least 20 AMPS ]
A microwave - fixed (like over range or in wall) it must have a dedicated circuit - moveable countertop models 20 Amp
Receptacles 20 Amp 
lights 15 AMP


Will one more 20 amp be enough ? No - you will need at least 3 more depending .


Do you have a sub panel that feeds your shop or in the shop ?
Do you only have a 20 Amp run from the main to the shop sub panel ?
What CB Amperage is in the main panel feeding the shop sub panel ?


If you have 20Amp CB in your main panel feeding the Sub panel in your shop - you will need to upgrade the sub panel feed CB upgraded, Wiring from Main to Sub panel upgraded, Sub Panel might also need to be upgraded.