I would have selected 3-5 but the option wasn't there. Its important to understand your character fundamentally but that can be done using several characters. Its just more work to pick up more than one character.
I main several characters and it has suited me well. I learn the characters moves, basics, staggers, mix, punishers, counters and MU's vs the entire cast(usually start with the top 10 MU's).
There are benefits to both options but I find it easier to counter specific characters by learning them fundamentally. It teaches me thier game plan, how they initiate offense and how they counter others.
Look at Sonic, he uses many characters and is very successful.
But I think loyalists can be effective its just that this meta is very counter heavy. Certain characters can shut down certain playstyle so some MU's might be very difficult.
FOr me to main one character only I'd have to have a character that has everything, pokes, staggers, frame traps, range, close up tools, zoning, gap closers and great dash and walk speed as well as nice combos And Damage and restands to top it all off.
Personally I think it's easier to use several characters. I use:
Nightwolf
Shang
Cassie Yaas Queen
Kotal both vars
And for specific matchups I use:
Sub Zero
Jax
Scarlet
Jade
Liu Kang Wu Shi Legend
Know how to use all these characters feels like its easier to play vs them and the entire cast. For example I hate using Cassie vs Shang but don't mind using Liu Kang vs Shang.