Do we have to make all the subsequent batches from very 1st mother culture batch?or we will be using previous batch for the next in a row?
You can do both.
If you use a backup of yoghurt you can reculture many times from that backup, but need to store it well to avoid contamination of the mother culture.
You also can do it back to back taking a few spoons from the ready and starting the new, however this increase the possible contamination.
You also can do the new batch from the mother for a while and then back to back and then using back up again - the only limitation is the contamination with other bacteria which reduce the viability in the long run.
If you boil the milk and sanitise the utensils and container with boiling water and also keep the mother culture well away from other product in the freezer you should be able to reculture a sachet for a very long time 
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