As others have said, a lot of the epics are a total of 19 but there are some anomalies such as Anky which is 18. The 7/5/7 food-interaction-toy is also not consistent across the epics, for example Smilo and Sino are 6/5/8.
What is really interesting is the point at which levelling up to 20 isn’t a big deal. For example Sino hits 6/5/8 at level 18 so you get the same DNA at 18 as you do at 20. And really for nearly every epic you are talking about 1 DNA difference between level 18 and 20.
Typically you see an increase in DNA gained from FIPing for epics at the 12 and 15 mark but again there are anomalies. If your alliance is struggling to get 20, then shoot for those bench marks.
And if you’re interested in rares, most give 54-55 DNA per FIP (Irri: 19/14/21; Argentino: 20/14/20; Spino g2: 21/14/20 etc. etc.) so just 4 FIP gets you over 200 rare DNA, which is a fuse in many cases. I can’t recall what commons give as we tend not to have them in our sancs but obviously it’s more.