@Umamaheswari_Yakkala As discussed, below are some of my high level non-technical scenario of when new ECID will be generated.
1. For the new customer, at very first time (Assuming the property has ECID cloud configuration)
2. At the time of migrating from legacy visitor ID to ECID, it will generate new ECID.
3. Any other existing ECID get expire as part of browser regulation (impact of ITP to Safari)
4. If cross-browser or cross device tracking not been implemented, then create a new ECID for the same customer.
Hope this helps you to get some fair understanding on non technical scenario.