I posted this question a long time ago...
To conclude this post, that's what I foudn after some reading and some Juniper support questions.
RFC 3768 state that :
3. VRRP Overview ../.. A virtual router is defined by its virtual router identifier (VRID) and a set of IP addresses. A VRRP router may associate a virtual router with its real addresses on an interface, and may also be configured with additional virtual router mappings and priority for virtual routers it is willing to backup. The mapping between VRID and addresses must be coordinated among all VRRP routers on a LAN. However, there is no restriction against reusing a VRID with a different address mapping on different LANs. The scope of each virtual router is restricted to a single LAN.
so the standard allows to use more than 255 VRID per box, with a limit of 255 VRID per broadcast domain.
*But* when I ask my Juniper support they said to me that using more that 255 VRRP groups globally on a EX4500 was not supported.
On QFX5xxx and last versions of JunOS, there's a nice feature that allows to run only one VRRP instance on a VLAN on physical interface, and have hundreds of dummy VRRP instance on the same physical interface, that don't exchange any data with the peers, but only inherit from the state of the "main" VRRP VLAN, where VRRP is really running.
I found that solution very smart.
Pascal