As such there is no minimum TDS limit defined for any form of water. The lesser the TDS the better.

Having said that, the minimum TDS based on some studies should be at least close to 30 ppm - keeps the mineral levels in check.

The Bureau of Indian Standards (BIS) fixes the upper limit of TDS in drinking water at 500 ppm. Crucially the standard also mentions that in case no alternative source of drinking water is available, then this upper limit can be relaxed to 2,000 ppm.

So technically maximum TDS obtained by RO filter should not cross 500 ppm and for health purposes it should be under 175 ppm. Also normally available domestic RO filters wont be able to handle water of more than 2500 ppm effectively - for that membranes of some other specification will be needed in line with RO membranes.

Hope this helps.