The closest way to tell it in French would be;

"Il a en banques dans les sept chiffres" (X XXX XXX (7 numbers))
or
"Il gagne dans les sept chiffres"

My last example is less precise as it can mean he earn that by year, or a win he raffled in the lotterie, while my first exemple mean he got that inside his pockets.

Millionnaire and Multimillionnaire could be used, but the term multi is not used the same way for everyone so it lack precision.

The lack of precision is like; "Il est riche" versus "Il est riche comme crésus". (the perception is not the same for everyone)

A note; The expression is used in French Canada, I can’t tell if it’s universal or not

You can see an example of the usage there on radio-canada.ca;

"Citant des dirigeants de banques, le journal soutient que certains bonus pourraient atteindre « six, sept, voire huit chiffres », selon les personnes. Selon les dirigeants rencontrés, le public américain pourrait être « abasourdi » des montants offerts."

In informal language, it is common to use the word peser to refer to someone’s (large) fortune.

Jeff Bezos pèse 180 milliards de dollars US.

Based on that, you could imagine some turns of phrases such as

Ces personnes pèsent chacune entre 1 et 10 millions.