I bought the top tier of service, which is about 15,000 units. BUT this has nothing to do with actual useage.
Next, I scanned two websites (once), not the 4000 available, and questioned if something was wrong, only to be chastised to contact support. (watch out their customer service is light on service)

OK, I did that, and that, and their response is very telling. We are exercising our refund rights and urge buyers to understand the limits are too low for most real sites, and for $199 MORE, you can fix that problem.

"The Link Scanner service is intended for small to medium-sized websites, so it's limited to 1000 pages and 50.000 links. So, the scan will stop once it reaches the 50.000 link limit, and you will see that error.

If you think the numbers on your site are lower, let us know so we can inspect it.

The scan limit will reset at the start of each month. However, you can always buy more credits from the dashboard.

We can also offer unlimited credits for an additional $199. In this case, you will never run out of credits. If this is something you're looking for, let me know and I can share a purchase link.

Kind Regards
Adnan Mujić from WebFactory Support Team"

So, I guess this may be helpful for very very small sites.