Ideas on generating membership numbers

We have 5 clients. Members have a prefix that identifies what client they are part of. For example, if a member is part of the first client, their membership number will start with an A. If they are part of the second client, their membership number will start with B and so on. Our concern now is that we will run out of alphabets to prefix membership numbers with when we have more than 26 clients. What ideas do you all have to solve this problem? We want it to be something simple because these are distributed to the end user, but we also want them to be flexible enough for our needs.