I sent this msg to tech support, but I figured I would try here too.

We have a couple servers with SM... do they do secondary DNS hosting (while we maintain the primary)? If so, for how much money.. and how many domains do we get to host?

If they don't do just secondary, do they do both primary and secondary? And
the same questions apply.

Thanks in advance!

They do offer DNS hosting for free. As to whether you want to use them exclusively or just as a secondary dns service, it is up to you.

In Orbit, you will have 'Domain Administration' under Network which allows you to do just that. You need to request them to add each and every single one of your domain name and you need to make changes to point one of your dns server IP with your domain name registrar to put them theirs.

Is there a limit to how many domains? I have dozens...

The only limitation is that you can only use their DNS service for servers hosted on their network and not beyond that. As for numbers, there is no limits.