There are some more advanced conntrack statistics available in /proc/net/stat/nf_conntrack.

For example, it includes insert_failed, which indicates conntrack has failed to create a new entry, which can be the root cause of some nasty networking errors. There is a full explanation of the fields available in the conntrack section of the lnstat tool.

The file itself breaks out every entry by CPU, but I don't think that's necessary - they can certainly be summed. Note that the entries column should not be summed.