Your network contains four Active Directory forests. Each forest contains an Active Directory Rights Management Services (AD RMS) root cluster.
All of the users in all of the forests must be able to access protected content from any of the forests.
You need to identify the minimum number of AD RMS trusts required.
How many trusts should you identify?
The number of AD RMS trusts required to interact between all AD RMS forests can be defined by using the following formula: N*(N-1).
Here N=4, so the number of trust is 12 (4*3).
References: AD RMS Prerequisites, Important considerations for installing AD RMS in a multi-forest environment