Zone within data Centre

What can be criteria of deciding the maximum number of zones within a data centre. For ex, If I have 30 Racks in a Data Centre, and if I have Primary and Secondary shards are 1,2 so minimum zones require will be 3. If each rack is having independent power supply and networking switch, then there can be 30 zones w.r.t power supply and networking failure. My question is what are the good and bad effect if number of zones are higher, how to decide upper bound of zone within a data centre.
Ta in Advance