This is a popular question. The INACSL standard focused on simulation design can guide you in this area.

http://www.nursingsimulation.org/article/S1876-1399(16)30126-8/fulltext

I would also refer you to the NCSBN published findings of their study.  You can retrieve the publication at:  https://www.ncsbn.org/685.htm  In this study two students were assigned to the role of the nurse. 

Faculty around the country dialogue on this question. The number of students per simulation scenario varies from 4-6 students per scenario. There is newer literature coming out on the role of the observer in simulation and the learning that does take place as an observer.

https://advancesinsimulation.biomedcentral.com/articles/10.1186/s41077-015-0004-8

 The key is how you structure learning for the observer role. Examples of this include assigning observers to watch for how the different concepts in QSEN are integrated or used (e.g. someone observes for safety, another communication/teamwork, etc.)