Stacey Collins posed this tough question to the last panel of the day at the BVR Divorce Summit. Kevin Yeanoplos said that he found "one perfect comp, I'd use it. It's not a matter of how many, but the quality of the data, and our judgement of how good it is." Collins agreed that "there's no magic number here."
Kevin recommends going first to the 8K's to look at the original source documents for transactions--because he fears errors in all the transaction databases. One word of warning; even services like EDGAROnline or 10K Wizard include data errors, so even this isn't a perfect solution.
Obviously "ask the owner, if available, if they know about any comparable deals; you won't get a good answer often, but when you do, it can be a fantastic comp," advises Collins.
And, watch out for announced deals. "How valuable is an offer for a company that never closed," asked Yeanoplos. "Very limited." Pratt's Stats only includes completed transactions, for this reason.