good practices when using gpg --symmetric?

[Robert J. Hansen]

All my remarks here are restricted to GnuPG.  Other cryptosystems will
have other answers.

julio baltazar wrote:
> - number and size of files: is there a difference (security-wise)
> between encrypting 10,000 small files or encrypting a single, larger
> archive containing all of them?

No.

> - file formats: should I be concerned about an attacker knowing which
> format a particular file is in? Files in a certain format typically have
> a magic number in the beginning or have other predefined content.

Not especially.

> Are there any other issues one should keep in mind to safely use gpg in
> --symmetric mode?

Impossible to answer without knowing more about your particular needs.
For the most part, the answer is "no."