Apache MPM configuration

After running /scripts/apache_mpm_calculator where should I put its output?
Also, is it reliable?

I hear it's a good estimate -- so at least a good starting point and you can fine-tune from there.
/usr/local/apache/conf/httpd.conf is the whole shebang or you could put it in /usr/local/apache/conf.d/limits.conf or create your own conf file in /usr/local/apache/conf.d/

https://www.techgeekbuzz.com/blog/nginx-vs-apache/

Yes, those comparisons are all well and good, but use whatever fits your use case. I have one server with 30+ customers on Apache, since it is normal and approachable to them and they can use .htaccess files. Another server is Nginx because it is about 5 sites under one customer and is heavily trafficked and tends to be PHP-heavy. Another server runs Apache for a single customer (1:1), running Magento. So use what fits best!