What % of tax-evaders would be critical mass?

[Subfallen]

If 100% of American citizens not employed by the Federal Government stopped paying taxes, obviously the income tax would be de facto dead.

But any x% too large to effectively prosecute would also kill the income tax. Because the dim-witted public would finally realize that universal laws can only be enforced normatively. There's not enough violence to go around.

So what is the smallest x%?