Is the client code or id unique across brokers within the exchange?
Each broker use different formats and length,
Zerodha - Alphanumeric and 6 characters
Upstox - Numeric and 6 characters
Kotak - Alphanumeric and 5 characters
are there any standard formats stipulated by exchange for client code like min-max length etc.?
I think it is decided by the brokers.
By client code, if you mean the login ID to your platform, then there is no standard format.
But your DP ID or Demat account ID is standardized. All CDSL DP IDs start with an 8 digit code which is the ID of the DP followed by a beneficiary or customer ID (BO ID).
So your Zerodha Demat ID will be 12081600 (Zerodha DP ID) followed by your own 8 digit ID.