Nobody knows how many Gardnerians there are, and figures in circulation deserve suspicion: reference works have given numbers in the hundreds with no dated source behind them. What can be said is that lineaged Gardnerian Wicca is small, a matter of covens rather than congregations, and very much smaller than the number of people who call themselves Wiccan.
Its influence is out of all proportion to that. Almost every modern Wiccan tradition descends from it or reacted against it. Alexandrian Wicca is a direct offshoot. Seax-Wica was created by a Gardnerian High Priest specifically as an open alternative to it. The eightfold festival year, the cast circle, the athame, the God and Goddess, the Charge, and the wording of rituals that appear in mass-market paperbacks with no attribution: those are Gardnerian, and they now furnish a religion practised by far more people who were never initiated than by those who were.
That success created the tradition's standing argument. British Traditional Wiccans hold that the word Wicca properly names the initiatory lineage and its offshoots, and that self-initiated eclectic practice, whatever its merits, is something else. Eclectics regard this as gatekeeping over a word. The dispute is genuine and unresolved, and it is about authority rather than about doctrine, which is what one would expect of a religion whose only institution is a line of descent.
Gardnerian covens exist in Britain, North America, Australia and across Europe. They initiate slowly, on purpose.