There have been a few threads trying to put a number on the eldar population left in the galaxy, with estimates ranging from the absurdly low, to very high. However, I haven't seen any that use quoted numbers in their extrapolations.
The first of two quotes I find to be useful concerns the battle between hive fleet Kraken and Iyanden. In that battle, "countless billions" died, totaling "four-fifths" of Iyanden's population. This quote puts the pre-battle population of Iyanden at less than one trillion, but much more than 100 billion, as 100 isn't "countless." For this, I'll take the easy way out and go with 500 billion, meaning that Iyanden now has 100 billion people left.
Iyanden is also said to have been the most populous craftworld before the conflict with Kraken, so we can put the other major craftworlds each at a slightly lower population; 450 billion for Alaitoc, 400 billion for Biel-tan, 250 billion for Saim-han, and 250 billion for Ulthwé.
From just the major 5, that gives us 1.45 trillion craftworlders. Adding in the 7 minor named craftworlds, if we set them at 125 billion each, we get an additional 875 billion eldar.
Adding in the "thousands" (we'll set that as two thousand) of minor, in-named, one liner, or bit piece craftworlds at a much lower 400 million each, we get 800 billion additional craftworlders.
Adding this together, the total population of the craftworld eldar would seem to be around 3.125 trillion. Very large, by earth standards, but also a tiny tiny population compared to the millions of worlds inhabited by humans (which would outnumber the eldar a thousandfold even if their average population was half that of modern day earth, or 3 billion).
It's also fitting with the "untold trillions" who died in the Fall, which a small fraction of eldar survived (3.125 trillion is a small fraction of even just a hundred trillion, or an even smaller fraction of 500 trillion if we use the same metric as at the top of the post).