The dome is removed by popping/prying it off the base, it's not threaded. The base has 6 small internal tabs, the dome has 3 large outer tabs, they just snap together. Rotating before popping may have a slight advantage if you can get three of the internal tabs to line up with the gaps between the tabs on the dome. Apply a little pressure to the dome, you will feel it move off
very slightly as you turn it. You might benefit from removing the fixture from the underside of the dash and unplugging the wiring pigtail from the lamp housing so as not to break the base and or rip the wiring if you get too ham-fisted.
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As for how many your specific vehicle should have, I couldn't help much there other than to say my '05 LJ has one in each footwell from the factory.