Cbarge for shared lines is what CME with UC520 supports, not regular Barge, under that premise, you have have upt to 8 IP Phones sharing the same line barging in to the active call. The corresponding amount of Cbarges will be directly associated to how many sessions you have configured for conferencing.
Below is a document that indicates how many DSPs a UC520 has, each DSP supports 2 conference sessions.
https://supportforums.cisco.com/docs/DOC-9744
Hope this helps!
