You will get good performance to energy density from a 15c ( continuous ) packs. When building your pack add multiples of packs until you reach matching discharge rate. For example 15c (2000mah) pack will pull 30a so you need 2p. You need to convert mah to Amin. So 4000mah will be 4ah or 240amin.

Then you divide by full throttle amp or 60A. 240amin/60a = 4min That means if you pull 60a continuous on 4000mah 15c pack you will get 4min of flying time.
If this setup is for 3d flying with 1/2 throttle hover, you can get between 2-3x that with good throttle management or 8-12min.

20c packs on the other hand gives you good performance but not enough energy if you match based on current draw. For example, new kokam 20c 3200mah willl pull 64a continuous so you can get by 1p setup but at 60a you will get little over 3min at full throttle. Not enough in my opinion. If you double up on 20c pack it defeats the purpose of using 20c packs.

go with Kokam 15c 2000mah packs