If I have multiple Regieleki VMAX with the "Transistor" Ability in play, will my Basic {L} Pokemon's attacks do +30 more damage for each one?
Yes, the Transistor Ability will stack.
Source: Silver Tempest FAQ; TPCi Rules Team (2022-11-10)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH12_58
SWSH12_58