What is better - having 10 databases for 10 organisations or 1 table for 10 organisation with org_id?
it all depends.

from my personal experience, if you have one database for each organisation, all the data is neat and clean and it is all in one place for each different organisation.

In case there are changes specific to a particular organisation it will affect only that particular organisation database - as the changes may or may not be welcomed for the other organisations.

On the other hand, it might cause extra overhead if for each new organisation you will need to create a new database, and that involves making sure you have space for backups, and scheduling time to run all the typical database maintenance routines, etc.

Basically different databases give you more flexibility (even the permissions set could be different) but it also might cause overhead.

There are strong points and not so good point in either approach.