Where can I find Admiralty number ?
sometime American ignore the Admiralty number,
Fort Gratiot Lighthouse,
https://en.wikipedia.org/wiki/Fort_Gratiot_Light
Grand Traverse Lighthouse,
https://en.wikipedia.org/wiki/Grand_Traverse_Light
Point Isabel Lighthouse,
https://en.wikipedia.org/wiki/Point_Isabel_Light
Portland Head Lighthouse,
https://en.wikipedia.org/wiki/Portland_Head_Light
Sand Island Lighthouse, (Wisconsin)
https://en.wikipedia.org/wiki/Sand_Island_Light_(Wisconsin) (https://en.wikipedia.org/wiki/Sand_Island_Light_(Wisconsin))
Sturgeon Point Lighthouse,
https://en.wikipedia.org/wiki/Sturgeon_Point_Light
I can suggest following ways:
1. You can check on amateur sites such as:
www.lightphotos.net (http://www.lightphotos.net)
https://www.unc.edu/~rowlett/lighthouse/ (https://www.unc.edu/~rowlett/lighthouse/)
http://listoflights.org/ (http://listoflights.org/)
2. You can find official data.
Admiralty list is commercial subscription (not available free of charge), but global data also available in NGA
https://msi.nga.mil/NGAPortal/MSI.portal?_nfpb=true&_st=&_pageLabel=msi_portal_page_62&pubCode=0007 (https://msi.nga.mil/NGAPortal/MSI.portal?_nfpb=true&_st=&_pageLabel=msi_portal_page_62&pubCode=0007)
Plus various local official lists.
Some of the resources you can find in Links page http://www.lightphotos.net/forum/index.php?action=links (http://www.lightphotos.net/forum/index.php?action=links)
Please note - admiralty numbers available only for active maritime navigational lights. I.e. for Great Lakes lighthouses, they have only US Coast Guard and ARLHS numbers.