Unclear what you mean, but if you want to have Google Merchant Center active on MANY sites - as opposed to just ONE site, then:

1. Each SITE must have its own, independent Google Account. You can only use ONE Google Merchant Center per Google Account, so if you have SEVERAL sites on a single Google login, you need to split them up.

2. If your sites are all stand-alone zencart sites, then just install the module for each one.

3. When installed (and when your sites have their OWN Google Accounts), configure each as is necessary - both at Google and within your site(s).