If you have started out with a batch of yogurt, don't worry! You can at least reuse the same yogurt for 5-7 days!
The presence of fermenting bacteria, Lactobacillus and Streptococcus is crucial for the fermentation procedure to continue. Due to this, even the tiniest amounts of microorganisms need to be present in the yogurt batch in order to reuse it again and prepare fresh yogurt out of it.