I was just about to do an analysis like this myself lol
The beholder section in Chapter 1 is "nearly 14 pages" from the Polygon preview, so given that it will start on around page 5, it looks like it will go to around page 18. Given that we're a bit into the giant section on page 23, either we go directly from beholders to giants, or we have at most 4 pages for something like dragons. That's still possible (Volo might complain that the good dragons refused to talk to him - presumably on Elminster's request - whilelthe evil ones just wanted to eat him), and give us a quick overview (quick history, pantheon, and role-playing tips), but I agree, that possibility is looking a bit grim.
We know that Chapter 1 will cover yuan-ti as well, which with the remaining mind flayer section and the orc section, would be enough to fill up the 30 pages or so. There might be another minor "featured" section two mixed in there as well, without a lair map, given that we didn't know about hags until now. Minotaurs, sahuagin, thri-kreen, troglodytes, and trolls would all be good candidates for such shorter sections.
The gauth is obviously in an overall Beholder section in Chapter 3, as you can catch the tail end of the Death Kiss section on the sample page. That means page 125 is relatively near the start of Chapter 3, probably at most 10 pages or so in, probably less. It looks like each "playable race" section will be about 1 1/2 to 2 pages long, so I would think a good estimate would be 4 to 6 full-fledged playable races after the firbolg, with a page for a chart for the "quick and dirty" races (unless this is either scattered among the monster info in Chapter 3 or in the NPC appendix)