The number in the second column of an entry in the output of
ls -la is the number of hard links pointing to the same inode; in the case of a directory entry this means the number of subdirectories including the special
. dir mentioned below; you can read more details e. g. in this answer.
.. represents the parent directory, which in this case is the root directory
/. 19 or 20 subdirectories of
/ is quite normal, as far as I can tell, but depends on the distribution and local additions.
. represents the current directory, in this case
/tmp - it contains only
.., as shown in the listing, thus 2 hard links: one for its sole existence in
/, one for
. pointing to it.
Your conclusion that
/tmp has "hidden" entries is wrong; the temp dir is definitely empty right now.
There are some oddities which I mentioned in my comments, but these are out of scope for this question. If you're experiencing any problems you should open new questions for them, with a detailed description what you see (don't forget to show output for evidence), what you expect and why you think it's wrong or unusual.