It is important to explain to clients that HRT is about transformation to their next highest level of order, which often requires several sessions for results to become apparent in their lives.
Often, others around them (partner, child, friend, boss) will see these changes first. They will not notice the changes until they actually "grow" to the next level. Therefore, three or four sessions is preferred.
You might want to offer four sessions with before and after voicings as a transformation protocol at a reduced price.