You can configure a maximum of 4 DNS server addresses in an IPv6 router advertisement.
device# configuration terminal
device (config)# interface ethernet 0/1
device(conf-if-eth-0/1)# ipv6 nd ra-dns-server 2001:DB8:0:ee44::1 lifetime 200
This example uses the ipv6 nd ra-dns-server command to configure a DNS server with the 2001:DB8:0:ee44::1 address to be advertised in a router advertisement for 200 seconds.