You can manage multiple companies under a single account. But they will share the same database. All contacts and contacts/leads of different companies will be on the same list. But you can manage them by adding specific tags.
Inside System, you can use tags to organize and filter your contacts and leads. You can set these tags manually or select them dynamically using conditions inside automation flows to tag and segment your contacts.
For example, you have two companies like, Company-A and Company-B. For 'Company A', you have a lead form (Form-A) to collect leads, and the same way, for Company-B, you have a form (Form-B). When your users submit Form-A, you set tag 'com-A' for that contact/lead with automation. The same process is also applicable for Company-B. And this way, you will be able to separate them.