Number of Zones for Zone awared Shard allocation

What can be the 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.
Tx in Advance

Please don't post the same question more than once, it makes it harder for people to help :slight_smile: