A public company has access to knowing how many accounts are in street name by a broker, they do not have access to how many people hold certificates, but they know how many certificates have been issued.
18,000 "investors" most likely means 18,000 accounts as a result.
I.E., if 9,000 individuals each held SLGLF in 2 separate accounts, the company sees 18,000 and so on.
I got this info from a Sr VP of another stock I own, and the figure was 2,869 June/2009 and 2,811 in June-2008.
I hold this stock in 3 accounts.
This is my understanding