There is no rule that I know of that requires an altar server to assist at Mass. Weekday Masses at my parish don’t have altar servers.
I think an altar server is required, for a Mass with a congregation, without a deacon, and without a concelebrant.
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html in the section “Mass Without a Deacon”:
“139. … An acolyte or other lay minister arranges the corporal, the purificator, the chalice, the pall, and
the Missal upon the altar. …
140. … The offerings of the faithful are received by the priest, assisted by the acolyte or other minister. …
142. After this, as the minister presents the cruets, …
145. After the prayer In spiritu humilitatis (Lord God, we ask you to receive us) or after the
incensation, the priest washes his hands standing at the side of the altar and, as the minister
pours the water, …”.
If there were a deacon or concelebrant they could do these altar server duties instead.