Technically, you need a single license to enable the feature. However, this does not mean that you will be compliant with Microsoft's licensing requirements. Best talk to your Microsoft representative.
Azure Cross-Tenant Setup
We are trying to establish a cross-tenant between two tenants, so to synchronize users from tenant B to tenant A. All users in tenant A (target tenant) have M365 E3 license assigned which means there is Entra ID P1 license already. But we only subscribed Business Standard or O365 E3 licenses for users in tenant B.
Understand that we need to subscribe Entra ID P1 for tenant B in order to establish the cross-tenant and the Entra ID P1 is charged per user. What if we subscribe one Entra ID use license in tenant B, are we able to setup the cross-tenant and synchronize all users to tenant A? Or we need to subscribe the Entra ID P1 license for all users in tenant B?