Minimum coverage needed of de novo small RNA sequencing

[hechanner]

Hello,

I am a novice member with some novice questions.
I have data about small RNA profile of an avian species without reference genome. I get less than 1M reads per sample after filtration. My questions are: 1, how many reads per sample should be enough? 2, for analysis, should I map them to an related species' genome?
Also, I am looking for publications about de novo in small RNA.

Thanks a lot in advance for your help!