You can read as many magazines as you like. If you spend all of your credits, you can buy new ones, and there's no limit to the number of credits you can buy. All back issues are included in your subscription and you can read as many as you like at no additional cost.
Yes, you may cancel your subscription whenever you like. When you sign up for a subscription you only pay for one month at a time in advance. You can easily cancel your subscription under "Settings" when you log in.