MY tech says three if not to high priced specially on clubbys cause semen quality isn't as good ,some times less if price is issue or if quality is good ...
If done properly and at the right time (when the cow is ready) one is sufficient. There's a grizzled old Cowboy lurking on here somewhere that has done a couple of flushes in the last 40 years that has always got it done with one straw for me. RCR