Q: Does it send my data anywhere?
A: No. It only makes a normal HTTPS request to each add-on's public xenforo.com resource page to read the latest version. No forum content, user data, or your site address is sent, and there is no third-party server.
Q: How many add-ons can it check?
A: All of them. There is no limit and no paid tier.
Q: How does it find an add-on's version?
A: From each add-on's own xenforo.com resource link in its addon.json (dev_url, support_url, faq_url, or extra_urls). Add-ons without a xenforo.com resource simply are not checked, and the home-page notice tells you how many that is.
Q: Will it email me?
A: Optionally. Administrators are emailed when a new update is found (you are not re-emailed about the same version). It can be turned off in the options.