I don't recommend many hygrometers and thermometers. It will drive you crazy sorting it all out. Lazy Gardener gave great advise to calibrate your equipment. Once calibrated it's accurate and then no need for more devices that will need different calibration numbers. Keep it simple.
If your unit has a thermometer assume it it way out in reading. You'd need to calibrate it to a trusted thermometer. Have faith in the medical thermometer in your bathroom. This is what you'd use to calibrate temp in lieu of hundred dollar laboratory calibration thermometers so have faith in it. Use it to find how much you need to adjust the incubator thermometer. Write the calibration down on tape and stick it right on the incubator. Get the medical to read 99.5F at top level of eggs (101.5F if a still air incubator) then subtract the incubator reading from 99.5 (or 101.5) for calibration number. ex. incubator reads 104.0; 99.5-104.0= -4.5. Write that on tape and stick to incubator as reminder to always subtract 4.5 from incubator temp for true temp (at top of eggs).
You can look up "salt test" here or anywhere online. Best resource is online cigar humidor site. It's simple and accurate. I'll list how I do one every spring prior to hatching season. My temp/hygrometer is a inexpensive small humidor model about 1.5 inches in diameter. It doesn't matter what you use as long as it works well and fits where you want it.
Pour salt in a milk or juice cap, add drops of water until saturated. I pour off standing water.
Put hygrometer and salt cap into sealed container. Sandwich or quart zip seal bags work well, I provide small pillow of air.
After minimum of 4 hours (humidity reading is stable) take the RH reading and subtract it from 75 for calibration number.
Ex. Your reading is 82%; 75-82= -7. Write that calibration on tape to stick to incubator as reminder to always subtract 7 from reading for true %RH.
If calibrated even inexpensive equipment is reliable. No need to purchase many units all reading different, just calibrate one and stick with it.
I highly recommend a low humidity incubation. I achieve the RH I want with either a double shot glass or small coffee cup sitting right in the incubator. I like 35%, you can decide what you'll use but know that the bottom of incubator trays will jump humidity to 60-80%. That's far too high to incubate at. The last three days called hatching (starting day 18) is when humidity is raised. I like it 70%+ but not too high it condensates all over the viewing glass, that's too high and a pain to see through.
Good luck, It's not as hard as all that writing makes it sound. Even chickens dare to hatch.