How can I count the number of reads that support a variant in a bam file?
If you don't mind a bit of manual counting, then samtools mpileup -f reference.fa -r chr22:425236-425236 alignments.bam will produce output where you can count the bases for that position. You could, of course, use the command line to do most of that automatically:

samtools mpileup -f reference.fa -r chr22:425236-425236 alignments.bam |
    cut -f 5 | tr '[a-z]' '[A-Z]' | fold -w 1 | sort | uniq -c


That'll give you a count of how many of each base were seen.