This is absolutely false. Exposure to a higher number of viruses might increase probably of an infection, but a single virus can certainly lead to a full blown infection in an individual.
(This makes sense in a simple thought model where the virus starts replicating once it "hits" you, and then your immune system needs N days to ramp up and attack it back. The bigger the initial load, the more the immune system will have to fight back once it's operational. - Not saying that's how it is, just that this is a simple model to make plausible what appears to be empirically seen.)
[1] https://www.erinbromage.com/post/the-risks-know-them-avoid-t...