On rhel 6.6 (mongodb was installed with rpm) the issue of ulimits, documentation says the following:
http://docs.mongodb.org/manual/reference/ulimit/#recommended-ulimit-settings

Every deployment may have unique requirements and settings; however, the following thresholds and settings are particularly important for mongod and mongos deployments:

-f (file size): unlimited
-t (cpu time): unlimited
-v (virtual memory): unlimited
-n (open files): 64000
-m (memory size): unlimited
-u (processes/threads): 64000

i.e open file and process ulimit (soft and hard) should be set to 64000.

In the mongod init script (/etc/init.d/mongod), the start function sets process ulimit to 32000:

  \\# Recommended ulimit values for mongod or mongos
  \\# See http://docs.mongodb.org/manual/reference/ulimit/#recommended-settings
  #
  ulimit -f unlimited
  ulimit -t unlimited
  ulimit -v unlimited
  ulimit -n 64000
  ulimit -m unlimited
  ulimit -u 32000

Here is a link to git :

https://github.com/mongodb/mongo/blob/400dabd20809c561d8de21438d4bb7c682129641/rpm/init.d-mongod#L60

While documentation recommends setting ulimit to 64000.

It is possible this is a documentation bug. If so, please move to DOCS.

Additionally, if /etc/security/limits.d/99-mongodb-nproc.conf file is created (as documentation recommends):

mongod       soft        nproc        64000
mongod       hard      nproc        64000
mongod       soft       nofile      64000
mongod       hard      nofile      64000

Then the ulimit set by the init script will be run over by the ulimit of 99-mongodb-nproc.conf file, and the ulimit will be set to the recommended.