Only you know if you want one or multiple monitors.
I used to own two monitors, a 22" and a 23". It's great for multitasking, but it's also distracting and I disliked looking at one monitor at an angle.
I now own one 27" monitor and I couldn't be happier.
Also, buy a monitor with a quality panel like IPS. Do not buy cheap crap with TN panels.