Let me be the first to welcome you to the Community. I’d like to share a few details for why the Set number of previous companies option is grayed out and not clickable.
As what @Lexiesmemere mentioned, you can only click on the Set number of previous companies once you have a company file open inside QuickBooks. To do that, follow the steps below:
1. Open QuickBooks and choose a company.
2. Log in with your company file credentials.
3. Once you’re already on the Home screen, click on the File menu at the top.
4. Choose Open Previous Company.
5. Select Set number of previous companies.
I attached a screenshot for your reference.
This information should answer your question for why you’re unable to click the Set number of previous companies option in QuickBooks Desktop.
Should you need anything else or if you have clarifications, please don't hesitate to let me know. I'll be here to keep helping. Take care!