If you have an Agency plan on SyncSignature, you will be able to manage multiple workspaces and hence multiple clients with your single account.
When you sign up, all your signatures are assigned to the only workspace you have with your account.
You can adjust your current workspace number of signatures by clicking on your workspace -> Settings from bottom left side.
That will make some signatures available to create new workspace. You can create new workspaces from bottom left menu.
You can always adjust number of signatures between multiple workspaces from the top right menu -> Accounts and then clicking on edit icon in front of any workspace.