What's the best way for a small business to implement Mailmunch to do email marketing?

I am a one man band small business and I would like to get more from email marketing. I am currently building up me newsletter subscribers and have emailed them two or three times since Christmas with special offers which resulted in a couple of sales. I don't want to become spam for my subscribers and I wonder what is the right amount of emails to send to people so that they don't think I am spamming them?