Hi,

First of all what you have to get an idea about is that, Odoo enterprise is not a separate thing, Odoo enterprise is a set of modules over the community edition.

1. Yes, as Odoo enterprise is a set of modules over the community edition.

Odoo community = Odoo community

Odoo enterprise = Odoo community + enterprise modules

 To install enterprise you need the community and community modules.

2. Yes there is no such a limit.

3. It purely depend on the hardware specificatiom you have.

4. Yes, you can change the db to Enterprise once you want it. Installing the module named web_enterprise change the community db  as enterprise db.

Thanks