How thick does an aluminum print bed need to be
Main factors that control the process of the print bed selection are


weight: too thick plate increases inertial force, limiting maximum acceleration/jerk (decreased print speed)
stiffness: too thin plate will warp when heated or bend during calibration (decreased print quality/printer reliability)


For table sizes around 400x400mm I would think of 4mm plate, but it still can warp if heated unevenly.

Sometimes it makes sense to use a sandwich-type table: lower level is MDF, cork panel for heat insulation and thin (1.5-2mm) aluminum heated bed on top.