Instruction of the Roman Missal in #211 states, "Mass should not be celebrated without
a server or the participation of at least one of the faithful except for some legitimate
and reasonable cause."
So ordinarily at least one person should be present when a priest celebrates Mass. The
law itself, however, recognized there can be a just and reasonable cause for a priest
to celebrate alone. And whether or not there is someone else present, the Mass would
Mere convenience or preference would not be sufficient reason to celebrate alone.