The honest answer to your question is, "it depends". Quail coveys intermingle and reorganize on almost a daily basis if coveys are in close enough proximity to do so. If that is the case, it doesn't matter over the long run how many you shoot out of a covey because they are genetically programed to seek a covey size of 13-15 birds. However, if you shoot them down to 10 continuously throughout the season, you can significantly reduce the local population until you are making the hunting harvest "additive mortality" where your take will ensure that the local population is reduced in the following year.
If coveys are farther apart, shooting the covey below any certain point will end up with a covey that is that small or smaller throughout the rest of the season. This could lead to a complete collapse of the local population.
If coveys reformulate toward the genetic goal throughout the season, there will come a point where it doesn't matter how large the covey is, any loss of members will result in the population decreasing in the coming year. The answer is that you can remove 40-60% of the population by whatever means (hunting, predation, accidents, disease, etc) without reducing the following year's population. There is no way to judge by covey size if you are nearing that point.
In the more immediate period, reducing a covey below 8-10 birds when the conditions require that they have sufficient numbers to retain heat through the overnight covey ring, will result in the possible total loss of the covey overnight. It is possible to start with a local population of 100 birds and never shoot a covey below 8 birds and still end the year with a local population of 8 birds ( as an example )! Good intentions don't make good statistics!