Originally Posted by Bombalurina
+1. A single tetra is a stressed tetra, and stressed fish are more prone to aggression and disease. I wouldn't consider getting bettas until you have 6 tetras, which will overstock your ten gallon somewhat.
+2 I had a school of 5 neon tetras, and I lost one to an unknown ailment (It may have been neon tetra disease considering the symptoms, fortunately it didn't affect any other fish since I removed the affected fish promptly) even with just 4, the remaining ones went into hiding in a corner in what could be described as cowering. I went out later that day and got two more and the school returned to normal.
5 is the minimum, 6 is recommended, highly!