What is the mandatory maximum on a first time 924 c charge and robbery charge

There is something called "mandatory minimum", we have not heard of a mandatory maximum. Offenders convicted under 924 (c) received an average sentence of 151 months which is about 12 years, however depending on the details of the crime and/or pertinent facts like an injury to a victim or heavy property loss. First time usually has very little to do with the sentencing as this crime is a very serious one.