Footnotes: is there any 'rule' on how to use them?
The rules will depend on:

1) Your chosen citation style

Your citation style will determine how you use footnotes. Some citation styles use footnotes as citations, where you'll find instances of Ibid and so forth. Others that utilise in-text citation state that footnotes are to only be used to add additional information not relevant to the main discussion (no citations). Your citation style will also indicate the formatting preferences. 

Regarding how many footnotes, I'm not sure what is the norm for a science-based paper, but generally for a social science stream you don't want to have that many footnotes.