If you work on 1cm of total fish length per litre lets round the tank size to 20 litres and say 20cm of fish.
What tetra do you want?
Many tetra are delicate and need a well matured tank post cycling of approx 5-6 months old imho. Neons especially.
If it were matured I'd go for a group of 5 neons or 6 or 7 green neons which are even smaller but just as nice.
Other tetra you'd possibly be a bit more limited to as all of them prefer to be in groups of 6 minimum to feel secure
depends on the kind of tetras you want personally i think 5g is to small as most are very active fish and need to be kept in groups
what about a betta?
regards scot