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.