Use a fresh one each time, unless you are travelling, and need to re use, only once or twice max. The more you use the needles, the higher the risk of infections.
I use one needle for a day e.g. 4 injections and have no problems whatsoever. As long as you remove the needle from the pen between injections the infection risk is just about zero when re-using. I wouldn't re-use one after a day's use. My reason for re-use is the cost to the NHS. Diabetes is an enormous cost to the NHS and I think we all need to control costs where we sensibly can.