According to Google the virus population on planet earth is 10,000,000,000,000,000,000,000,000,000,000 so there's a lot of mutation going on and we're still here. So random mutation of one more well-intentioned virus that kills cancer probably isn't a threat.
That's a population number. COVID alone accounts for 10^11 virus particles in a single human during peak infection, or about 10^19 in total. That is getting close to the 10^31 total.
But to be precise about all viruses; A over-the-thumb estimate is 10^24 viruses of the total population of the earth's biosphere are actively living in the collective biomass of the human species at every moment. That is merely the population of viruses currently alive in all humans, ranging from the ones going hunt on bacteria in our gut to the ones living in our blood and flesh.
If we estimate the weight of a virion particle to be 1 femtogram (which is roughly the dry mass of a covid particle), that yields you 10^6 kilograms of biomatter. About the weight of a Saturn V rocket.
So not a couple dozen out of the number OP posted. It's a significiant number.
And you have to keep in mind that OP's number does not only include the human virion particles but all virion particles in nature, including megaviruses, virions, virophages, bacteriophages etc. Most of those are not compatible with how human bodies work, probably 99.999% of viruses in nature will not survive in a human body, they're specialised to hunt other things.
By introducing a few nanograms of virus material to a human, you're contributing effectively 0% to the natural virus levels in just a single human, in the grand view of things it does not matter. And the moment these viruses escape, they're subject to evolutionary pressure, meaning they will evolve to not kill their hosts very quickly or perish before they get a chance.
And even, to become a threat, the virus may need to mutate several times just to become even a little contagious. Most viruses are not really contagious (as in respiratory contagious viruses).
