It is completely up to you what number you choose as your reload amount, most decide to choose a number that is over how many of their customers have opted in to receive SMS text. This helps to ensure that you always have enough credits to send an SMS message. (
Campaigns will show you how many of your customers are eligible to receive and SMS Text, you can also use
Recipient Lists to generate a list of Opted-In SMS Customers which you can refresh to get an accurate number whenever needed.)