I ran 4x TC20 at the same time (because I had no heroes I wanted to level at that point and I really, really wanted 5* heros since I had ascension materials for 3/5 of the colors.
I would not suggest to do it because you are limited to the heros you currently have but I got lucky and got Sartana of my 7th pull and was hyped to “not have much to do for a month”.
The other thing is you could get some good 4* out of TC20 aswell. I’d pull heros as long as I can ascend them.
If I was in your position I’d go for 3 or even 4 TC20s for 5* heros but again, if the games gets to boring for you because you don’t level and get “nothing done” I’d go down to 2 TC20s, one for cheap and the other one for backpack heros.
Results since Feb 2018.