The answer is a definite - "it depends".  Tons of what if's and how intertwined the companies are. Such as accounting, payroll, physical location, products or services, and on and on.  You probably could combine them but I bet if you ask your accountant they will want everything kept separate if for no other reason than legal issues.

I see that you have started the beginner series and you are asking Expert level if not Developer level questions.
First and foremost I recommend you get a good solid base down now with the beginner and expert courses.  Don't try and directly design your db yet.  Follow along with the courses first.
Then , get the yellow pad out and start identifying what data you need and what tables you might need.

Watch the Relationships video first.
I can think of at least 10 others that I could point you to.

Point is you have to learn crawl before your ready to run a marathon.

Way more advanced and NOT a teaching course is Richards Access Business & Contact Database.  There is GREAT info there but it is assumed that you know a good deal about Access.

Phil, you generally don't make separate tables for the same KIND of thing. You'll have a company table, an employee table, a workstation table, etc. Then you'll use relationships to join everything together. Keep learning. You'll get it.

Thanks Scott and Richard, there's a lot to discover, Relationships and ABCD certainly cover what I need, no netflix for me for a while :) thanks