I'm no expert but I have a lone budgie and he's perfectly happy. I think that if you are around outside of working hours and keep them in a room you spend a lot of time in, a lone bird should be ok.
It does however depend on what your budget and level of commitment is. For example, if you're happy to throw some money at it but don't have too much time, you should get two. On the other hand, if you have a smaller budget but more time, you could be better off with one.
It also depends on what you want out of the bird(s). If you want more of a companion that you can train without distraction, you could be better off with one. If however you are not interested in training and only want some company, you could be better off with multiple.
In answer to your final question, as much as you can spare. This is a lot easier if they are kept in a central room with activity.
If you start with one, you can always get a another if need be.
Good luck
