18993 is your user ID on Mathematics. The email always lists your main stackexchange.com profile for the merge procedure, not a per-site profile, which will use your network account ID that will pretty much never be the same as an on-site user ID.
I'm confused what exactly you're getting caught up on, though. Are you only clicking that link? There are two separate emails you need to verify, one gets sent to the email address for each account. You need to click both of them in order to confirm the merge. If you don't have access to one of the email addresses anymore in order to retrieve the second email, then you'll need to use the contact form and use the other option to get a hold of a human to do the merge manually. You cannot complete the merge yourself with access to only one email.
This wasn't immediately obvious to me, but your merge is failing because one of your accounts has profiles on it that are considered high-reputation. We don't allow the normal merge process to complete in this case, and after confirming the first time your request was dumped into a queue that only gets checked once a day in most cases. I've approved your merge and you should be good to go.