https://learn.microsoft.com/en-us/entra/identity/users/directory-service-limits-restrictions
Most relevant from that is, under the Applications section:
A user, group, or service principal can have a maximum of 1,500 app role assignments. The limitation is on the assigned service principal, user, or group across all app roles and not on the number of assignments of a single app role. This limit includes app role assignments where the resource service principal has been soft-deleted.
which means that any single object can be directly assigned to a maximum of 1,500 app role assignments (Users and Groups blade on an Enterprise Application). In the service limits article, I don't see mention of a limit on the maximum number of app role assignments that a single application can have linked to it, which matches my understanding that there is not a limit.
For SCIM provisioning of group objects, there is no limit from Entra's end on the number of group objects that can be provisioned into an application.