One of the main "sparks" to lead adults to discovery/identification/diagnosis as autistic is that their child or grandchild was diagnosed, and in learning about autism to better support them, they come to realize that they're on the spectrum themselves. Some celebrate that, some accept it, and yet others go into denial.
So it's not that there aren't already a lot of autists, but rather, that many are hidden in plain view, living out their lives without realizing they're on the spectrum.
And let's also keep in mind that many autists choose to keep a low profile - not mentioning it at all, or only in a select few, as there is still, regretfully, significant stigma and potential professional / employment and social repercussions from disclosure, as many of our fellow forum members can attest.