You need a total of six bulbs (LED or otherwise) for the back lighting. Bulb size is '194' and is a 'wedge' type.
That's what the incandecent version looks like. LED variatons all look a little different but the base is always the same.
You can buy them on ebay, amazon, or even walmart now. I personally bought mine from superbrightleds.com and I love 'em.
Link to the ones I bought. Oh, and for reference, a typical incandescent 194 is 25-30 lumens.
A couple of tips.
Some LED bulbs may be polarized, meaning they only work when put in one direction. If at first one or two don't work, flip it around. The ones I bought are not polarized, meaning you can put them in any way you like and they'll work regardless.
Go for LEDs with a wider 'viewing angle'. Viewing angle refers to how the light is tossed around. the wider the better, otherwise you can end up with what are known as 'hot spots', where some areas are brighter than others; which not only hurts your eyes in some cases (or gives you a headache) but can also make the gauges hard to read.
If you plan on changing the color of your cluster to say, blue for example, (assuming yours is green), you'll need one of two things. A, a brighter bulb or B, to remove the faces and sand the color off. The bulbs themselves, stock, are clear bulbs with no color but it is the cluster's face that filters the light. I don't recommend sanding the color off the gauges as you loose all accuracy, due to the needles not being keyed. Although it is doable. Here's a pic of my cluster. If your cluster is green (or any color really) and you buy colored LEDs matching this (IE, green cluster green leds, blue cluster blue leds, etc) then this isn't a problem.
If you're curious, the HVAC (climate controls) take the same type of bulb, but only two. Unlike the cluster, the HVAC (in most cases) has a filter that is entirely removable inside it. Standard ranger HVACs (except for the silver ones) usually have this filter.
Of course, there's more than one way to put LEDs in the cluster, but 'plug and play' leds like I've been talking about are by far the easiest way and the most cost effective. There is more than one way to 'skin the cat' so if you want more options to consider then feel free to ask. I'd be happy to help.