As many as you're going to use?  Ive got it on my desktop at the office and at home, some years I put it on a laptop as well.

So, I can load it on my desktop and 3 or 4 laptops?

Yes, but be very careful about e-filing from more than one and getting the right backup from the proper computer and not overwriting an existing file with an old back and ....

Only Efile from one system though, otherwise each time you check for acknowledgements it pulls all of them for that EFIN into the computer youre on at that moment and then the other computers wont see them....it just gets confusing.

Is this for multiple employees?   You'll want to set up user accounts for each person, with their own log in and password.

https://proconnect.intuit.com/community/manage-access/help/manage-users-for-proseries/00/4963

Thanks for your help.