You can simply make a short list of projects and a time estimations, combined with how this project fits in. The simplest will, probably, be a short summary of the projects, the time they still take up and/or the stage they're in. Keep in mind for this you don't have to list other clients names, these usually are seen as confidential unless specified otherwise in contracts and it could hurt your business to put these out into the public.
For example something like the following, it lists your time but it doesn't show what or from whom, just generically, you're still working on 1 project for an estimated 30 hours and you have 7 hours 'maintenance' every month:
4 hours every month, photo editing
3 hours every month, updating website
~30 hours left, custom project, final stages
What it sounds like is that your client is looking to get the 'guarantee' you won't be working on those or that they're really small or insignificant compared to what they're doing with you. They're looking to ensure you're the right company/guy to make sure their project hits the targets of (mostly) time in this case, since your attention won't be on other projects.
Depending on the type of project and relationship you can tackle this also in another way, where you sit down with the client, you look at what is requested and depending on the type of work you make a certain commitment: it will be done before X. If you want to be more agile, you can say: I will spend more than 30 hours a week on it (or if you have more people, you can say 60 hours etc).
Your client is asking this for a reason, it's probably smart to get that reason out in the open.
