It really depends on how busy the sites are... I personally know people who have 50+ low traffic sites on a 2GB VPS instance (including cPanel - MySQL, Apache, Mail + DNS services).

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.

It all depends on how much traffic those websites get and what kind of content they host. You can indeed run 50 low end websites with low trafics from a droplet with 2GB that won't be a problem.

While its depend on use case, i suggest you to get 2 medium droplet. One for Database and one for website(nginx+X)

6 separate droplet will increase your management cost also! Think that your problem will be 6x more.

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 are lucky to have web site load at different time spans 1 small droplet for all sites 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

@faud.nahdi Are you sure about that?

https://www.zippykid.com/2013/02/21/do-you-need-a-dedicated-ip-address-for-seo/