It’s not permitted to say the mass without at least one other person, except in special circumstances.
From the current GIRM:
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.
- 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.
- 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.
Just and reasonable is a moderately strong bar… not insurmountable, but still.
But as for hermits, most hermits actually
live in community, simply praying alone, working alone, and eating alone. They come together for the prayers of the hours and the mass.
The Carthusians are monk-hermits… and the bulk of the modern hermits!
newadvent.org/cathen/03388a.htm
en.wikipedia.org/wiki/Carthusian
Where is “just and reasonable” defined? If a priest wishes to celebrate Mass alone, and there’s no one around, that seems reasonable, and it certainly is just. It’s not as if he’s avoiding people to celebrate Mass.