March 29 (Reuters) - Kotak Mahindra Bank
* Exec says seeks to double customer base in 18 months
* Launces new digital banking product called '811'
* Exec says bank branch expansion will be more measured
* Exec says estimates stressed assets in indian banking system net of provisions to be 14 trln rupees
* Exec says have nothing specific to announce about any acquisition, looking at all options Further company coverage: (Reporting By Devidutta Tripathy)