Approximately, how many families have been identified?
I've often often come across figures for the total number of species on Earth. Recently, I found myself wondering about the encompassing ranks above them, specifically family, but I can't recall any figures on family and it's a little difficult finding what I want by typing "family" into a Google search. ;)
The NCBI Taxonomy statistics page displays the following information:
There are currently 73540 genera, 331418 species, and 23127 taxa of higher order. Since the number of taxa decreases with the genericity of the taxon, there are probably around 20000 families, give or take a few thousand.
The 2011 paper How Many Species Are There on Earth and in the Ocean? indirectly answers this question as well as any other source you'll find I imagine. It estimates how many species there are total based on the rate of discovery of higher taxa; it includes plots of number of taxa over time for the major groups of life in Figure S1. Which gives:
Animalia - 5300 families in 2011 (the plots are given with only one significant figure, the second one's my estimate) (estimated total: 5800)
Chromista - 270 families (estimated total: 360)
Fungi - 550 families (estimated total: 620)
Plantae - 750 families (estimated total: 800)
Protozoa - 280 families (estimated total: 310)
Archaea - 27 families (no estimated total; the number has been increasing exponentially so far)
Bacteria - 300 families (same as for Archaea)
Which gives us a total of 7477 families in 2011, with an estimated total of (ignoring Archaea and Bacteria, who don't really fall in the same kind of classification anyway) 7890 families. (make that 7500 families discovered by 2011 and 8000 estimated in total given the imprecision involved in my reading the plots).
The Plant List has 642 families listed: http://www.theplantlist.org/1.1/about/#changes.
For a quick comparison, Wikipedia lists
This only covers a subset of the total, however (see here for other animal classes). It is relatively difficult to find a good, and relatively complete answer to this, though.
I don't know about other groups, but about plants, number of families depends on the system you follow. Recent version of The Plant List (1.1) estimates about 352 000 species of Angiosperms and lists over 400 families. See http://www.theplantlist.org/1.1/browse/A/ It is very good and reliable source of information. Second very good source about plant systematics is database Tropicos provided by Missouri Botanical Garden and Angiosperm Phylogeny Group. See list of families according to APG3 http://www.mobot.org/MOBOT/research/APweb/ and click to "Families" on the top of the page. Just note, that there are also synonyms, so that You can't just count the rows. ;-)
I'm adding this as another answer since it's a different source, but Gaurav gave this excellent resource as an answer to this related question:
The Integrated Taxonomic Information System (ITIS) is maintained regularly by a consortium of North American governmental agencies, and will give you a list of all the classes, orders, families or genera in any of seven kingdoms they recognize. It might be biased towards North American taxa, but it might be quicker to get a list from then rather than extracting them from Wikipedia.
