Winged...What is considered a larger number? Is it proportional to the number of rats that you'll see spontaneous deaths?

The reason I ask is I just came back up from checking on the rats and another blue had passed away...This one wasn't eaten or abused in anyway...He was in a complete different container with only 4 other rats like the other...I'm losing all my blues and those are the ones that I'm trying to hold back and keep...

From what I'm hearing this is for the most part normal, I'm just seeing more and more sensitive to is since I just recently started breeding rats? Especially since they full access to quality food, water and aren't crowded in their living areas...