Hi @iuliachiciudean,
Your demultiplexed summary is showing you only have a single sample. I don't think this is correct, right? You may need to revisit your previous steps to sort this out first before moving forward.
You just need to do a bit more reading on the forum 
. This is probably one of our most frequently asked/answered questions. One example here, and here. One note, min overlap requirement for merging has been changed from 20 to 12nt in DADA2.
In my first response above I shared a link that answers this in great detail, please have another look there.
The difference between 124 and 120 is probably negligible, so for my simplicity I imagine the tutorial just uses 120. Much like the pirate code, the "median 20 cut off point" is more like a guideline, not a rule. So as per the need of your data you can adjust that as you want.
But you should certainly try and figure out why you only have 1 sample first.