There are currently 2,079,142 members of the military.
The current percentage of the population that identifies as “transgender” is .5%, or .005.
This would result in an estimate of 10,395 if the “averages” are applied. 15,000 sounds high, but if the government was paying for the surgery and medications…those who were transitioning may have self selected into the Military.
But, over estimating it by 50% seems a little “excessive.”
The .5% number was a “guesstimate”, not a survey result. I think I read somewhere (I looked for the source, but can’t find it) that the LBGT group that created the number based it on a percentage of their own group which probably biases the number upward.