How many rounds of interviews is best?

As many as it takes to make an informed decision regarding the candidate's fit for the role... how is that for a non-answer? 

But seriously, this is going to vary wildly depending on the role, the need, the company, the candidate... there are so many variables that we can't be prescriptive here. 

BUT - for the vast majority of roles, when following the HireBest approach, you should be able to get the data you need to make a decision entirely within the Decision Round (hence the name!)

The Screening Round and First Round are important though, so you don't waste time conducting extensive Decision Round interviews on a candidate who doesn't meet the basic criteria for fit against the Cipher. 

So, at a minimum, a screening round, followed by a decision round, possibly with a first round in-between in some circumstances. 

That said - don't forego an extra round if needed to have confidence in your decision. The cost of a bad hire far outweighs the cost of a couple extra hours of time upfront.