Linux users are typically configured with a maximum number of files to be opened. This is describe at
based upon your installation method.
To monitor how many open files a user has one can run
lsof -u <user>
replacing with the linux username who started the Neo4j process.
To understand the limits for said one can also run
replacing with the linux processID for the running Neo4j process.
This will produce output similar to
cat /proc/5219/limits Limit Soft Limit Hard Limit Units Max cpu time unlimited unlimited seconds Max file size unlimited unlimited bytes Max data size unlimited unlimited bytes Max stack size 8388608 unlimited bytes Max core file size 0 unlimited bytes Max resident set unlimited unlimited bytes Max processes 31357 31357 processes Max open files 40000 40000 files