Do I need to refill sealant after puncture?
Pull the valve core and poke a 2mm Allen wrench down to test the fluid level with the valve at the 6 o'clock position with the tire off the ground. You want "some" free liquid. How much depends on the tire size but I usually look for at least 3mm. 

Some amount of sealant from a new installation goes to coating the tire and in many cases filling in the porosity of the casing. So if you find there's no free sealant and you're surprised you lost that much, that might be why.

How much free sealant you want is largely based on preference in terms of flat resistance versus weight. It's not a super exact science. 

With the valve core removed you can inject more sealant using a commercial injector or various improvised tools.