That "/vmfs/volumes/verylongnumber" is true datastore name, auto-generated by esxi.
And "/vmfs/volumes/DatastoreName" is just link pointing to it, easier to remember and type.
You can see it in output of directory listing if you issue "ls -al" in "/vmfs/volumes".
There is no need to type that long number (but you are allowed to do it), you can still address your datastore with "DatastoreName".