You should use a signature instead of just adding your link at the end of every post to start with, build say, 10-20 for the first month every day, then increase it slowly, you want a steady steam of backlinks which increases at a steady rate.
Actually there is no limit, but be under rudder... don't leave your link at any spammy site, this will decrease your page rank at search engine. Give some quality backlink to your site everyday.
Building backlinks should be an ongoing part of your SEO duties. Submit to no more than 20 directories a day.