Let's see what the giant section titled WEALTH & PRESTIGE MARKERS on page 7 of the rules says:
The six wealth markers numbered 7 through 12 and the three prestige markers numbered 4, 5 and 6 can come into play during the
game via various card effects.
When first entering play, a wealth/prestige marker must be placed
into any one of the vacant gray circular spaces around the outskirts
of the city grid.
When a card effect allows one of these markers to be moved, it must
be moved to a vacant (different) gray space, if any.
If all nine gray spaces are filled, no further movement of these markers is possible—ignore any card effect that would otherwise do so.
The rule book is your friend! In this case, they dedicated an entire page to just these couple simple rules, so it was really really easy to find.