There are a few ways to set this up and a lot of it depends on how much you want to spend overall vs how much manual setup you want to do. For instance if you were to set up a couple droplets, get licenses and install WHM/cPanel and WHMCS you would have a turn key setup but would have to pay monthly licensing fees for the software. WHMCS does support many other control panels and there is even a third party plugin available to allow you to resell droplets directly as VPS servers. With this setup you can also configure WHMCS to use multiple hosting servers and provision accounts appropriately.
There are a few other hosting backoffice solutions like WHMCS, some are good but quite a few are short on features or rather buggy.
If on the other hand you do not mind getting your hands dirty and setting up new accounts manually you could choose to use one of the many free web hosting control panels on your droplets and add customer accounts to them manually when a new user signs up.
It would also be possible to go bare-bones and do all the configuration manually but if you are reselling shared hosting I would strongly recommend using a control panel on each droplet to manage the resources and provision new accounts.
As for resource usage, this will depend quite a bit on your customers. If you will be marketing to users who will usually not do much more than set up static websites with low to moderate traffic you could host quite a few on each droplet you run but if your users are more likely to be running complex scripts or applications to power their sites the average resource usage on each account will be higher.
I would not recommend using anything smaller than a 2GB droplet (or a few 1GB droplets) to get started. For a shared hosting environment the 512MB droplet would not provide sufficient memory to handle the full LAMP stack required.