I'm not an expert, but base system installation - MySQL, Apache, Mail etc - will eat about 200MB of droplet's memory and a few GBs of the disk so I would vote for a single, big enough droplet.
Still depends on your requirements.
The short answer 'No you should not create separate droplets for each website (unless you know you have to)'.
As you ask this question it looks you haven't done any performance testing and tuning of the sites. You may start with one droplet, deploy the sites, do performance testing and tuning (6x times less configuration, administration and etc) and adjust droplet size. When finished you can decide whether it would be better for you to pay for N droplets + more administration or 1 droplet + possibly higher price in total for bigger droplet. Don't forget one droplet might be more efficient for load balancing and if you are lucky to have web site load at different time spans you 1 droplet for 1 site will fit you best.
If you use 1 droplet for multiple websites, it means a single IP address is shared by multiple websites which are not good for SEO. Check this why a dedicated IP address is important for each site http://www.colocationamerica.com/services/why-a-dedicated-ip-address-is-important.htm