50
It would be a massive shit show and would collapse rather quickly. Aside from how we've seen them 'contain' places like Portland, showing they don't have the skills they'd need to do it on a small scale, they literally don't have the manpower. From The Military Journal of Operations:
In the Winter 1995 edition of Parameters, James Quinlivan wrote an article (‘Force Requirements in Stability Operations’) arguing that force ratios are the best method for determining the appropriate force requirement. Quinlivan determined that a force ratio of 20 troops per 1,000 of the population is required for successful stabilization, which he defined as “[creating] an environment orderly enough that most routine civil functions could be carried out.”[ii] While Quinlivan’s study included peaceful post-war occupations, counterinsurgencies, stabilization, humanitarian and peacekeeping operations, he was the first to advocate that force ratios based on a country’s population were the best predictor for a successful stability operation.
For a population the size of the US at (rounding up to) 350M people, that requires 7M troops. Even dropping the 20% of people who don't live in or around big cities, you're still looking at 5.6M troops, and a fuckload of uncontrolled / uncontrollable land perfect for counter insurgency tactics. Given that the us has about 2.12M active and reserve troops, they'd have to more than double their ranks to be able to stand a chance at containing the urban areas of the US, and that's not counting anything else they need to do.
