Since I posted the question I found the answer.... here it is for those who would like to know the answer:

There is no upper limit on the number of PIs, although the involvement of each PI should be justified by the aims of the project. In most cases, single project grant applications using the multiple PI option (e.g., R01, R21) will have two to three PIs depending upon the work scope. A proposal that involves PIs that appear to have questionable qualifications and expertise or an insufficient contribution or role will not fair well in the review process.

In addition, anyone designated as a PI must meet the following criteria: The Principal Investigator, or Project Director/Principal Investigator (PD/PI) is defined as the individual(s) judged by the applicant organization to have the appropriate level of authority and responsibility to direct the project or program supported by the grant. The applicant organization may designate multiple individuals as PD/PIs who share the authority and responsibility for leading and directing the project, intellectually and logistically. Each PD/PI is responsible and accountable to the applicant organization, or, as appropriate, to a collaborating organization, for the proper conduct of the project or program including the submission of all required reports.The presence of more than one identified PD/PI on an application or award diminishes neither the responsibility nor the accountability of any individual PD/PI.

Also, all applicants proposing team science efforts are strongly encouraged to contact their NIH program officials at the earliest possible date to discuss the appropriateness of the multiple-PI model for the support of their research.

Anna

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