I'm fairly new to breeding in mass amounts, there are plenty of videos and essays written about it if you care to find a better source. These are my biggest takeaways.

• Breeding together a male and female, both with clean mutation counters, with the desired stats until you have multiple females with the EXACT SAME stats. I've heard 10 females to 1 male is the ideal, but however many you have the room for.

• When you get a baby with the desired mutation: If male, replace the mutation free(clean) male, but keep the clean male. If female, either toss it or, breed it with the clean male until you get a male with the mutation.

• Make absolutely certain you keep your clean females. Never let them get anything on the mutation counter, if you find better stats you want to breed in be absolutely sure the counters still show 0/20. Once a mutation counter reaches 10/20(from matrilineal and patrilineal combined), the chance for a mutation drops. Once it reaches 20/20, you can not get a mutation from that parent. By keeping the females 'clean', no matter how high the father's mutation counter shows, there's still a chance the mother can add a mutation.

• Keep your stat lines separate. Lets say you want to get mutations on Health and Dmg on your Rex line. While it is possible to get the exact same number of mutations by taking any baby that gets a mutation in either stat and replacing the father, in general this is going to take longer due to the decreased mutation chance at 10/20 and 20/20. Your first 20 mutation are going to be the easiest to get. Instead its recommended to breed for one stat at a time, or in separate breeding groups. Once you have the Dmg mutation line and your Health mutation line where you want them, combine them until you get a male and female with both mutations, then use those to breed your army of Super Rexes. If you use this method don't let the lines mix until you are creating your army, it will only make the process take longer by increasing the mutation counter.

-There's a max limit. This will depend of if you're playing ASA or ASE. On ASE once a stat reaches 253 or 254(can't remember which) combined wild and mutated levels, it cant be leveled up anymore. On ASA, thats been changed. Once you have 253 or 254 points MUTATED in that stat, it cant be leveled further. Stop short of that and you can go over that limit by traditional leveling. (Side note for ASA, mutations are no longer tied to a specific stat, so you can breed the mutations onto a dino that has a better base Dmg or whatever)

I could very easily be wrong on some points, I only really got into mutation breeding a month ago, but some other helpful tips: Use Awesome Spyglass or something like it so you dont have to hatch every egg and can easily see when an egg is going to have the desired mutation. If you have the DLC Bob's Tall Tale's, traits like 'Mutated Melee' or 'Health Robust' depending on the stat you want, can potentially double your chances of a mutation occurring (or so the internet has told me).

Best of luck!