Yes, you can change number of users at the time of your next renewal cycle. You can add additional users any time during your ongoing billing cycle. However, you can reduce number of users during your next billing cycle.
No, you need not to buy different subscriptions. With the same subscription plan, you can setup intranet for multiple companies. There is no limit on the number of departmental sites within any company. However, plan restricts total number of named users across all companies.