How many days is the PEth positive after one beer

Too many variables to give a general answer.

I don't want to gaslight you here, but as Curlyben said, there is too little information to form an accurate response.

Prepare for your next test by NOT drinking at all, and avoid anything with Alcohol in it. Then you won't be panicking.

Its too late for this test... whats done is done.

After drinking one 12 oz budweiser, how many days will the serum PEth be positive?

Nobody will tell you because its YOUR job to stay free of alcohol as long as you are being tested. And its against the site rules to help anyone cheat or get over on their testing.