Although I don't know the aswer but I would assume dhcp relay's limit is differnt from DHCP server's, which takes memory to do stateful operation.
I read somewhere the number of server IPs you can configure in a dhcp-relay was 8. And I would guess if there is any limitation exist, it would be the number of interfaces instead.
I don't think it's written anywhere. So you need to open a TT at TAC to ask. You might get a similar answer from them.