@NickQuinn If the goal is to get a broad sense of how many members are using signatures, the quickest option may be to create, but then not deploy, a signature badge.
In Community Admin -> Users -> Badges, you can create a badge that would award anyone who has input a signature:
When inputting the rule, select "Advanced Editor". You can then use this formula:
setting.profile.signature != 'default'
When you save this badge, you'll then see a tab that estimates how many users qualify. This is the estimate for the number of users who have set a signature.