Why does the number in the account name appear?

[h-nn-ng]

I have just synchronized my Fastmail account with my Pixel smartphone via DAVx⁵ for the first time.
All contacts show up, so everything is great, but why # 7? :)

[rfc2822]

https://manual.davx5.com/accounts_collections.html#address-book-accounts

DAVx⁵ address books accounts are named after their assocated CardDAV address book plus the account name plus an ID to make the account name unique.

Accounts must have a unique name (requirement by Android; the unique key of accounts is their name), and without ID there could be duplicates.

Internally, #7 is the collection ID. You can see it in the debug info in the Collections table.