zero. keep drinking enough water and you will be fine.

The likelihood that a stone has developed and is causing these symptoms
when no stone was seen 2 months ago is very low.

If you had symptoms in November and had a negative CT scan it's unlikely the symptoms you're having now are from a stone. Stones can form fast but are still unlikely. Problem is that a CT with contrast can miss small stones because the contrast is white and stones are white on CT. If blood is in the urine it's possible. More likely cystitis. Check into a test by a company called Pathnostics. It checks PCR for a lot of microorganisms that may not grow out on culture.