There answer seem to be different depending on the method of collection, but there is lots of literature on the subject, so Google and PubMed searches will be very helpful for you. I searched Google for "amount blood zebrafish" and found lots of good information in seconds. For example, see https://www.ncbi.nlm.nih.gov/pubmed/?term=Repeated+Blood+Collection+for+Blood+Tests+in+Adult+Zebrafish which states that for one method "decapitation with scissors by cutting through the pectoral girdle" can yield between 5–10µl of blood, so that would mean between 50-100 adult fish if you need 500µl.