12,000 appears to be a total market figure for independent ("community") pharmacies in England.
If you look at the FAQ section on the PharmAssist website they state they have 600 users online... the website also appears to have not been updated since 2017 so not sure what is going on there.
I have sent the company an email to inquire about how many clients they have as there is no mention of 12,000 anywhere on their website.