Context: the number of people illegally voting in the United States is somewhere less than an average suburban high school's senior graduating population. The GOP just likes to inflate numbers because they want to make it harder to vote, not to make it easier.
It's quite a bit more than liberals claim, and quite a bit less than conservatives believe.
The problem is illegal voting happens all the time, and in so many different forms.
It can be voting it two states, voting in a district/state/precinct that isn't yours, voting when not allowed to vote, etc. many times its accidental, many times its malicious, but it is also something so hard to actually uncover. Id bet at least one person who regularly comments has illegally voted, most likely without thinking about it.