A carrier can't apply for an MC number without an existing USDOT registration, and authority doesn't activate the moment the application goes in. Three things have to clear first: a BOC-3 process-agent filing, a BMC-91 proof-of-insurance filing, and FMCSA's mandatory public protest period — typically a few weeks in total.
Not every carrier gets one. Private carriers hauling their own goods, and carriers hauling certain exempt commodities, operate on a USDOT number alone and never apply for MC authority.