Hi La-Qa,
1. As far as I know there is no limit on the number of tenants that you can create. However, you need to consider whether the multi-tenancy function is the best solution for your needs. If you are looking to multi-tenancy as a way to provide different fields for different regions within a single organisation (as your second question suggests) then I would suggest that multi-tenancy might be an 'overkill' in that scenario. Please read How Multi-Tenancy Works - CA Service Management - 17.1 - CA Technologies Documentation and review all of the other documentation concerning multi-tenancy before proceeding.
2. There are two approaches to hiding or enabling certain fields for particular tenants or regions:
a) use 'PDM_IF' within the form to show or hide a field depending on which region or tenant the ticket belongs to.
This approach can result in complicated forms which require more effort to migrate across upgrades, but it consolidates all the changes within a single location and needs no other action to implement it.
b) create a 'Form Group' for a particular tenant or region.
Define Form Groups - CA Service Management - 17.1 - CA Technologies Documentation
Each Form Group has its own folder (e.g. site\mods\www\htmpl\analyst\Region1) , where you place the tenant or region's version of a form. The form can show exactly the fields that the tenant or region needs. You associate the Form Group with a Role (using the field 'Customization Form Group'); then you allocate the Role to an Analyst who services the tenant or region. When an analyst switches to a role which has a Form Group attached to it, SDM looks in the Form Group folder for customised forms before looking in the default folder. Best practice is to group Roles into Access Types, and allocate an access type to a contact, but it is also possible to allocate individual roles directly to a contact (although I tend to think this is more complicated to administer). This approach can reduce the need to complicate forms with 'PDM_IF's, resulting in simpler forms which are easier to upgrade.
Hope that helps you with your consideration.
Regards,
James