Although the Sizing Tool shows the ability to enter a given number of external database nodes, at this time only one CAPC is supported in CAPM environment, which would have one external MySql database only.
Release : 3.7
Component : IM Reporting / Admin / Configuration
CAPC 3.7 added the possibility to externalize the MySql server, as explained in the documentation:
However, at this time only one MySql server is supported in CA Performance Center. Clustering with MySql has not been tested.
So, although the Sizing Tool wizard have the choice to enter a given number for the "Number of external PC Database Nodes" option, actually only one external MySql node is supported.
Reference for the Sizing Tool: