GIRM 115-198 covers Mass with the people. Sections 199-251, concelebration. Starting with numbered section 252, we come to section III of GIRM Chapter IV. Here and through the twenty sections that follow, the Church regulates “Mass at Which only One Minister Participates.” I’ll give you a taste:
252. At a Mass celebrated by a Priest with only one minister to assist him and to make the responses, the rite of Mass with the people is followed (cf. nos. 120-169), the minister saying the people’s parts if appropriate.
253. If, however, the minister is a Deacon, he performs his proper functions (cf. nos. 171-186) and likewise carries out the other parts, that is, those of the people.
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 instructions, and the blessing at the end of Mass are omitted.
How to interpret “just and reasonable cause”? Clergy can pray the Mass virtually anytime, anywhere, and it doesn’t strike me that the Church has much need or interest in poking too deeply into this.
GIRM 255 covers brief preparatory notes, 256-259 the introductory rites, 260-264 the Liturgy of the Word, 265-271 the Liturgy of the Eucharist, and 272 the concluding rites. If anybody would like to delve deeper, speak now or hold your peace.