The Amount of eggs has nothing 'really' to do with it. Your Temp Will Fluxuate a little but the more eggs, the warmer your incubator will stay, and the less it has to run to maintain that temp.
You are also using a Still Air w/o out a fan, Temps ARE Going to be hard to control, and them get right. But at the same time, there Could be something wrong with gauge..
When Incubating eggs temp Should be 99.5... lowest is 78.5 and highest 101.0... NOW this is the Kicker... to low, you have more losses and they do take longer to hatch out.. Too High..again you will have more losses, and the hatch out a little sooner.. 99.5 IS the Ideal Temp for Incubating... However, with mine, I do have a Still Air with a Fan and Egg Rotator.. Actually the BEST WAY To Go.. Fan and Egg Rotator... so get a.s.a.p... Continuing On... I get mine as close to 99.5 as I can, but I dont let my temp get any higher then 100.5. and no less then 85.5 (which I Hate running it at)... Even If it take me a couple of days of tweeking BEFORE I place my eggs.. I get my Temp. up..
It is also better to run your temps a little on the low side, then on the High side. In case things go wrong. You will lose you eggs faster to over Heating, then getting to cold.
I use 3 different Temp gauges..that I pay $8. for at local Pet store.. or
Petco has them.. Mine is NOT digital, not does it have kind of temp gauge on it as My Incubator is an Old one.."Little Giant 9200" I use two Digital Temp gauges outside and a 'standard' type inside, plus 2 Humidity Gauges.. This in case one fails, or is just not reading right..
Again, Dealing with a Still Air w/o a Fan is not easy, the Fan does help out a lot.. also Not have to remember to rotate your eggs at least once day it Great too..
And last but Not least.... EVERY ONES Incubator does run a little different, so getting to know your Incubator will take some time.. BUT the temp you should try to aim for is 99.5..