I'm working on a project management for productive purposes of Prosopis spp fruit for the preparation of flour those fruits. The project is in a part of Argentina in communities of small producers engaged to collect and grind the fruits of Prosopis.
Dear Giacomo. According to some articles consulted and asking other researchers the sample should not be less than 300 cases. It all depends on the study population.
Perhaps some references like these could be of help:
http://idbdocs.iadb.org/wsdocs/getdocument.aspx?docnum=1441488
https://ideas.repec.org/p/van/wpaper/0046.html
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.134.5664&rep=rep1&type=pdf
http://econweb.ucsd.edu/~rcarson/papers/gscope.pdf
http://ageconsearch.umn.edu/bitstream/31005/1/22010044.pdf
You may want to consult the orientations given in online resources such as Netquest.com or Survey Monkey (https://es.surveymonkey.com/mp/sample-size/).
Best wishes
What is the ideal sample size for applying the Contingent Valuation Method in the analysis of willingness to pay? - ResearchGate. Available from: https://www.researchgate.net/post/What_is_the_ideal_sample_size_for_applying_the_Contingent_Valuation_Method_in_the_analysis_of_willingness_to_pay#584ecc2ecbd5c2dda4371d3c [accessed Dec 12, 2016].