Most of us would need more than 100 domains; amazon says to contact them if you need more.
Hopefully that's just to get a better price; do you really want to pay Amazon $100 every month to host slave DNS for 100 domains? I sure don't.
Either get yourself a small VPS, and use my Master2Slave DNS Replicator, or even buy a small VPS with a DirectAdmin license and use multi-server option; it should be a lot less expensive and give you more control.
Or contact me; we've do a lot of slave DNS for thousands of domains for lots of DirectAdmin users, and have servers in the US and Europe, at much lower prices. And there are other contributors here who offer the same or similar services.
Jeff