At the moment, with your current stocking, I would not add any bettas.
Two problems.....
1) Your danio. Danios prefer cooler temperatures, between 18-22*C whereas bettas prefer their temperature at 26-30*C. If the water is cooler than the preferred temperature for the betta, it will become lethargic. If the temperature is above the preferred temperature for the danio, the danio will have a short lifespan. Another problem is that danios are nippy fish, that could harass your betta, danios are a lot more nippy when they are not kept in proper school sizes <6.
Either rehome your danio or up your school number to at least 6 and don't get a betta.
2) Your skirt tetra. Black/white skirt tetras are very nippy fish, especially when kept alone or in small schools. If you get a betta, your white skirt tetra could harass your betta all that time, possibly stressing her to the point of death.
Your neons also may be nippy, unless you get at least three more.
With not having had bettas before, if you rehome your danio and white skirt tetra and up the school number to at least 6 of your neon tetras then I think it would be best if you only get one betta until you're more experienced.