I'm not sure he has a number.
This page lists all the World Grand Prix Racers from the films, each of whom has a colour scheme and a number. This list includes people like Lewis Hamilton (No. 2) and Francesco Bernoulli (No. 1).
However, this other Wikipedia page lists the racers in slightly more detail. For Jeff Gorvette (No. 24) it states:
The character does not appear in international versions of Cars 2. In
two scenes (one where Lightning meets him at the party in Tokyo, and
another when that scene appears in Mater's nightmare), Gorvette is
replaced by a completely different character, voiced by a regionally
better known racer than Jeff Gordon. Most of them share the body shape
with Max Schnell and the least of them share after the body of Miguel
Mark Winterbottom as Frosty (Australian release)
Alonso as Fernando Alonso (Spanish release)
Vitaly Petrov as Vitaly
Petrov (Russian release)
Jan Nilsson as Flash (Swedish release)
Unknown as Long Ge (Chinese release)
Memo Rojas as Memo Rojas, Jr.
Sebastian Vettel as Sebastian Schnell (German
Claudia Leitte as Carla Veloso (Brazilian release)
Unknown as Raoul ÇaRoule (French release)
No number is listed. Some further searching led me to this site, which is one of many selling toy replicas of the cars in the films. Unlike most of the other racers, no number is listed against Vitaly's entry, or his picture:
Additionally, stills from the film show nothing:
Therefore, I think it's safe to assume he most likely doesn't have a racing number, as he is only feature in one country's version of the film.