I'm hosting websites on Plesk with: memcached + eaccelerator + apache deflate + apache cache + reverse nginx + php cgid

Is it possible to keep running this server with 100 or more websites without lag?

My semi-qualified guess would be yes, if...
- you got rid of plesk
- replaced apache and php cgid with nginx and php-fpm
- used nginx built in cache mechanism to replace memcache
- use redis server
- traffic is anonymous
- shared code base as in all 100 sites running of the same wordpress core.

100 websites with 100 anonymous unique visitors per day is only 10K hits which I doubt would be concurrent. But if most of the conditions suggested above are not met, I would say you'd require double the cpu and mem.

Now I'm running 8 GB ram with 4 cores. Server configuration is the same. So about how many websites I could host?

You can host an almost infinite amount of sites. The biggest deciding factor on your resource usage will be the type of sites and how many concurrent users you're getting. There is no end all be all formula to figure it out.

For a control panel on low-memory servers, you might want to check out ServerPilot.

The ServerPilot agent uses 20MB of memory. Other than that, there's usage by your system services: Nginx (2MB), Apache (10MB), MySQL (45MB), and PHP-FPM (35MB). PHP-FPM will then have an increase in memory usage based on how many sites you are running and how busy those sites are.