I have a reseller account with a host, where they offer 1TB of bandwidth... Do I need to divide this up to work out the bandwidth for clients? or do I offer each client 1TB of bandwidth? Or if I have 20 clients do I divide the 1TB over 20?
That depends on the control panel you using. From your previous posts, I guess you are using Plesk control panel. So, you have to divide 1TB for your clients, you can't offer 1TB BW for every customers.
You can host many websites using the reseller account, but the total BW you offer shouldn't be more than the total BW allocated to your reseller account.
Create a hosting package and then setup a domain template according to the package, that will help you to decide the total number of domains you can host on your reseller account. Less the resources you offer, more the domains you can create.