For Denso plugs with 14mm plug threads, flat seat washer - torque can vary from 18 - 22ft.lbs. NGK plugs of the same size and setup have torque ranges a little less. I'm pretty sure they tell you to turn some faction of revolutions after the gasket makes contact. Atleast, that is what my replacement NGK plugs said on the box. Part of that is the threads can get covered with oil and/or owners smear anti-seize or the built-in coating can throw off the actual torque ratings. Compression of the gasket, after it makes contact with the head with x factional revolutions of the plug is less sensitive to those issues.
If you want to use a torque wrench, with that low of a torque, most of those wrenches will not work. Need a special wrench (ie, more expensive) to get the proper torque. Rule of thumb, most torque wrenches are most reliable/accurate from the bottom 20% to top 80% of their range. So a wrench that is spec'd to run from 25-250ft.lbs, may only accurately resolve torque ranges from 60ft.lb to 205ft.lbs. Not gospel truth, as even though their are not 100% reliable at their extreme ranges, they can be close enough for this.
I haven't had to use a torque wrench on my plugs in some time. With experience, you'll develop a "feel" for how tight bolts need to get - so called muscle memory. Plus - the box will say turn an additional X fractional revolutions.