the max level of a relict depends on how many trials the student/ apprentice has completed, if a student is given a higher tier wand, then they also gain more trials

to level up a relict up to the max level you can have a staff or student attune to it at the attunement table, once its attuned the relic level will slowly climb towards its max level (combat speeds this up)

relics gained from a battle crystal are always capped at the same max level (i think that was 16)

a student that has completed all 5 trials (T3 wand) will create a lvl 21 relict on graduation, hiring or apprenticeship
an apprentice that has completed all 5 trials will create a lvl 24 relict on graduation or hiring
the same student will create another relict of the same level if they became staff and are retiring

I have not found a way to create a higher lvl relict then 24, though I dont think there can be a relict much higher then that anyway (since mages can not equip relicts much higher then their own level (+7 in the library upgrade))