how many ntp clients are supported?

Windows Server 2008 R2

i'm starting to sync our Solaris servers time with the domain time and i'm using my "PDC" as the NTP server for the domain and for Solaris. my "PDC" in turn syncs with some time servers located in on the net.

questions:

1. can i configure a second NTP server in my domain in case the primary one goes offline?
2. how many NTP clients can DC NTP server support based on good practices?

regards,

Rino