Lutheranism is usually put at somewhere between seventy and eighty million adherents, which makes it one of the largest Protestant families. Where those people are has changed completely within living memory.
The historic heartlands are statistically hollow. Germany and the Nordic countries retain very large formal memberships, in Denmark, Finland, Norway, Sweden and Iceland the Lutheran church was until recently the established church and enrolment was close to automatic, but weekly attendance across the region generally runs in low single figures as a percentage. Memberships in Germany and Sweden have been falling by hundreds of thousands a year, much of it through formal resignation, which in Germany carries a tax consequence and is therefore recorded precisely.
The growth is in Africa. The Ethiopian Evangelical Church Mekane Yesus is now the largest Lutheran church in the world, with a membership in the many millions and still rising, and Tanzania, Madagascar and Nigeria hold further large bodies. That shift has already changed the internal politics of the Lutheran World Federation, because the African churches are broadly conservative on sexuality and ordination while the European and North American churches that fund much of the federation's work are not.
In the United States the pattern is a merger followed by a split. Most moderate and liberal Lutherans combined into the Evangelical Lutheran Church in America in 1988; its 2009 decision to permit clergy in same-sex relationships led several hundred congregations to leave and form new bodies. The ELCA has lost roughly half its reported membership since its formation, from a combination of that departure, deaths and general decline.