If you get NOC from employer u can change end number of times.

Incase if you are looking for transfer than minimum you have to complete one year of service with every employer.

The difference in Transfer an NOC is completed different

In tranfer you do not need to go back for medical,or exit the country it can be done locally.whereas in case of NOC you have to exit the country and comeback on fresh visa ,do the medical,fingerprints and so on

Best of luck

Regards
Felix