BPMN spec suggests usage of pool as a boundary for a process. Specifically for internal private and abstract processes.
For an internal process - the mapping to a single process is an obvious choice. The whole goal of defining an internal process using BPMN is to be able to capture the sequence of steps required to accomplish a particular business goal. In terms of mapping an internal process definition to an executable notation. BPEL is an obvious choice for mapping to execuatble. It is industry standard for orchestrating services and human interaction (even more so with bpel4people spec).
For abstract processes again mapping a process to one BPEL4WS abstract process is the recommended approach. Even though the current version of spec does not delve into the mappings.