Q. What licenses are required to use Azure AD Connect Health?

A. Azure AD Connect Health is part of Azure AD Premium. For each monitoring agent (e.g. AD FS, AD DS and Azure AD Connect) a certain number of Premium licenses are required. For the first instance a single license is required, for each additional agent 25 additional premium licenses are required. This is documented at https://azure.microsoft.com/en-us/pricing/details/active-directory/.