Q:

How many transcription factors are there?

In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow (or transcription) of genetic information from DNA to messenger RNA.
My question is simply how many transcription factors are there in the genomes of: 

Humans
Anopheles gambiae (or it's close taxonomic relatives)
Less prioritized: any other species

A:

This is the Curated database of mouse and human transcription factors. And this is the paper in which they describe how they curated the database. In summary, there are 3230 putative mouse TF, 1200 of which are described in scientific papers.

A:

As the number of human TFs have been discussed by the previous answers, I'll limit myself to Anopheles gambiae.
In the same issue of Science in which Holt et al. published a genome sequence for Anopheles gambiae (1), Zdobnov et al. published a comparison of the A. gambiae and Drosophila melanogaster genomes.(2) While the two species diverged about 250 million years ago, and the D. melanogaster genome is about twice the size of the A. gambiae genome, the two species still show broad similarities and a similar number of proteins.
The authors identified 6089 protein pairs as "clear orthologs", corresponding to 47% and 44% of the of the Anophelesand and Drosophila proteins respectively. One might thus be tempted to estimate the number of TFs in A. gambiae as similar to that in D. melanogaster. For the latter, the FlyTF database (http://www.mrc-lmb.cam.ac.uk/genomes/FlyTF/old_index.html) currently lists 753 putative site-specific TFs and 454 "well supported" candidates. More recently, Hammonds et al. identified 708 melanogaster genes "likely to encode sequence-specific DNA-binding TFs".(3)
Edit: You may also want to check out the following question on Biostars: http://www.biostars.org/p/53590/

A:

Pretty interesting question. There are a few numbers out, I remember that round about 10% of the human genes are coding for transcription factors (unfortunately I don'T remember the source for that).
This paper from 2002 estimates between 2000 and 3000 factors:

Signal transduction and the control of gene expression.

This paper from 2009 states a number of 1391 manually curated factors and also speculates about a number of 2000 - 3000 factors in total. This is definitely interesting to read.

A census of human transcription factors: function, expression and
evolution.

