Optimal settings for the Nginx + php-fpm + mysql bundle on a virtual machine 1 core + 512mb?

Question

PHP

I set up ubuntu 14.04 on virtualbox, installed Nginx + php-fpm + mysql bundle, installed wordpress. Everything seems to be ok.
But there are a lot of questions:
- under such a system, how many sites can be launched and which ones?
- what kind of load can such a virtual machine withstand? how to monitor load? top command for load average and memory loading or are there better tools?
- how to do load testing? there is jmeter but it didn’t work for me, there are a lot of settings.
- how to configure everything for maximum performance? there are a lot of articles on google and everyone advises different settings.
- Do I still need to put Apache in the backend?