Your plan includes a set number of companies you can monitor at any one time through your Credit Risk Manager. This is not a monthly allowance that resets, it's a concurrent limit. Once you've reached it, you won't be able to add new businesses to monitor until you free up space or increase your capacity.
Your options:
Remove a company: go to your Credit Risk Manager and remove a business you no longer need to monitor. This frees up a slot immediately.
Upgrade your plan: the Pro plan lets you monitor up to 10 companies at a time, Enterprise up to 100. Upgrading gives you more capacity on an ongoing basis.
Add a Credit Bundle: if you're on Startup plan, Pro plan or an Enterprise Plan and need to monitor more companies than your plan includes, a Credit Bundle lets you top up your capacity without changing your plan.
You can read more about our plans and credit bundles here