A client needs over 15 SSL certificates for various subdomains on 2 machines - an IIS 6 and IIS 7 one. They want to buy 2 wildcard SSLs (1 per box) as it'd be more cost-effective. The question is, though, is that as normally you can only have one IP bound to a single SSL cert, do all these subdomains still have to be on different IPs or can they be on the same one (per box)?
He needs 1 cert per 2nd level domain. So if he has just subdomains of example.com, then he would only need 1. If he has subdomains of example.com and example.net, then he would need two; and so on.
The same cert can be used on as many servers as he wants (with the aforementioned limitation concerning domains).
For HTTPS, you need 1 IP per content site. So if all the various subdomains are completely different sites, then yes they all need 1 IP each. If some are the same, then they can share an IP.
SSL cert doesn't contain any IP information, so there no IP checks based on SSl certs for http
No. They can all be on the same IP address. If they are on different IIS sites, you just need to set up SSL Host Headers: http://www.sslshopper.com/article-how-to-configure-ssl-host-headers-in-iis-6.html