You generally need to make a lot of pulls from TC20 before getting that first 5*. I’ve been running TC20 non-stop now for about 9-10 months so that’s about 150ish pulls. At 5% I’d expect 7-8 5* heroes from that many pulls on average.
Some get more in that number of pulls, some get far less. I actually pulled close to double the expected. But initially I pulled a LOT of 3* and occasionally a 4*.