What box do I get the active duty pay deduction from on my w2. my state allows me to deduct active duty income

It would be the total amount in Box 1 of your W2.  

Everyone on active duty receives base pay. The amount depends on your rank, and how many years you've been in the military. 

My husband served active duty for PART of 2020. The other part, he served as a "reservist" or NOT active duty. How do I figure out the ACTIVE DUTY portion when working on the Active Duty Pay Deduction? 

Check the LES, check orders, calendar, whatever you can you find to prove active duty. See How to read a Reserve and National Guard Leave and ... - DFAS Field 45 has the state taxable amount which may not include the active duty.