From the General Instruction of the Roman Missal (GIRM):
“97. The faithful, moreover, should not refuse to serve the People of God in gladness whenever they are asked to perform some particular service or function in the celebration.”
The “pastor or rector of the church” decides who is to be an altar server, in the absence of an instituted acolyte. From the GIRM:
“107. Liturgical functions that are not proper to the Priest or the Deacon and are mentioned above (nos. 100-106) may even be entrusted by means of a liturgical blessing or a temporary deputation to suitable lay persons chosen by the pastor or the rector of the church. As to the function of serving the Priest at the altar, the norms established by Bishop for his diocese should be observed.”
Instituted acolytes are discussed in n. 98 and 187-193. Also from the GIRM:
“100. In the absence of an instituted acolyte, there may be deputed lay ministers to serve at the altar ….”.
Some rubrics from the Order of Mass in the Roman Missal that highlight that there should not be a Priest alone in the sanctuary for the Mass are:
“1. When the people are gathered, the Priest approaches the altar with the ministers while the Entrance Chant is sung.
When he has arrived at the altar, after making a profound bow with the ministers, the Priest venerates the altar with a kiss and, if appropriate, incenses the cross and the altar. Then, with the ministers, he goes to the chair.”
“21. When all this has been done, the Offertory Chant begins. Meanwhile, the ministers place the corporal, the purificatory, the chalice, the pall, and the Missal on the altar.”
Another task requiring a second person in the sanctuary is described in the GIRM:
“145. … the Priest washes his hands standing at the side of the altar and, as the minister pours the water, says quietly, Lava me, Domine (Wash me, O Lord). "