Is reading /dev/urandom thread-safe?
As long as each execution of the function is in its own thread (i.e., the local variables number, urandom, bytes_read are not shared between threads), I don't see any thread-safety problems. Each thread will then have its own file descriptor into /dev/urandom. /dev/urandom can be opened simultaneously from multiple processes, so that's okay.
By the way, /dev/urandom can fail to open, and your code should deal with it. Some causes are: running out of available file descriptors; /dev not properly mounted (although in this case you have bigger problems); your program is being run in a special chroot which denies access to any devices; etc.