I’m interested in this because Discourse is commonly underrated in usage and market-share statistics that I’ve seen.
Describing all websites using Discourse as “customers” is not accurate as it is only an assumption by builtwith.com. “Clients” or “users” would be better terms because they allow for both non-financial and remote relationships.
The numbers at builtwith.com could be confirmed because they list them all if you’re prepared to pay for them:
16,773 live + 10,895 past = 27,668 historical websites
16,773 live + 72,108 redirects = 88,881 current URLs
This appears to be a more complete list than sites like similartech.com (9,303), wappalyzer.com (3,700) and whatcms.com (1,354). But all of them only count public-facing instances so the many private intranet and home server instances won’t be counted by any of them.