Message Blasts cost 1 message credit for each person added to the Message Blast (ie. a Message Blast with 20 clients would cost 20 credits). Your plan includes monthly base message credits that match the plan appointment limit (if your plan includes 75 appointments/month, you get 150 free credits each month). If you run out of your base credits within a month, you can purchase Paid credits for $10 for 250 credits.