I think you are mixing up a couple of concepts. Way oversimplified, viral mutations are the result of random chance. So, the more copies of the virus that are made, the more likely that it will suffer a mutation that will change some feature of it. So, it is true to say that the more people infected, the greater likelihood of a mutation. That should not be conflated with treatment resistance. When a pathogen is treated, say by an antibiotic, the antibiotic will wipe out the vast majority of the pathogens. The ones that aren't killed likely weren't because they are partially resistant to the antibiotic due to mutations. Those resistant ones, if you don't finish the treatment, can multiply and be passed on to someone else. They also get antibiotics, and kill all but the most resistant bacteria. This goes on until, eventually, we have inadvertently bred antibiotic resistant bacteria. That is where mutations can make a treatment less effective.