License required for EP read/write to R3 operation

I read some message here regarding EP - R3 License. I'd like to get more clear license requirement for my case.

In our region, we hardly find clear response from SAP on this issue.

My scenarios is typical : Many User Account in EP UME (either local db or link to AD, but not in R3), and we have lots of custom Web Dynpro application using JCo destination to read as well as write data to R3.

Let's say we have 5 concurrent EP user having "WRITE" operation (via our web dynpro and then JCo, RFC) against R3.

We're not sure "how many" R3 License needed ? one communication user account license ? or 5 dialog user account licenses? or 5 ESS/MSS user license ? or ...?

Hope somebody can share his experience.

Thanks