We should also mention that with systemd you can update /etc/systemd/system/mongodprod.service.d/limits.conf on the following note on the https://docs.mongodb.org/manual/reference/ulimit/#unix-ulimit-settings page:

NOTE
Red Hat Enterprise Linux and CentOS 6 place a max process limitation of 1024 which overrides ulimit settings. Create a file named /etc/security/limits.d/99-mongodb-nproc.conf with new soft nproc and hard nproc values to increase the process limit. See /etc/security/limits.d/90-nproc.conf file as an example.