From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“III. MASS AT WHICH ONLY ONE MINISTER PARTICIPATES
. …
254. Mass should not be celebrated without a minister or at least one of the faithful, except for a just and reasonable cause. In this case, the greetings, the introductory or explanatory remarks (
monitiones), and the blessing at the end of Mass are omitted.”
This section of the GIRM is from 252 - 272, giving a detailed description of this “Mass at Which Only One Minister Participates”.