There are 5,392 SNPs reported by dbSNP for BRCA1. You can find a list of them here (http://www.genecards.org/cgi-bin/carddisp.pl?gene=BRCA1) along with their position on the chromosome. From there you can export the list, sort by chromosome location, determine the range of chr 17 positions for each exon, count the number of SNPs that fall within that range for each exon and then you will know which exon has the most SNPs. There are many resources that can help you find the location (chromosome position of each exon), such as UCSC but there are many others. I hope this is a sufficient amount of information for you to be able to generate an answer to your question. Good luck!

https://genome.ucsc.edu/cgi-bin/hgTracks?db=hg38&position=chr17%3A43044295-43125370&hgsid=453150535_GLWRRi3aYF4PMH488pSkghnTBskq