how many stats per generation ?

How many stats per generation can be gained when feeding regular treats ? If I start with 3-year-old of 100 stats, then 20-year-old would have about 800, 900 ? How much gain should be expected then from second generation between 0 to 20, assuming regular treats would be given ?

Regular Treats give 1-2 stats a day so over it's lifetime (3-20 years) a horse can gain between 119 and 238 stats from treats alone. 
Using a riding school average of 2-4 stats per day they could gain between 238 and 476 stats. (this could be more or less if shows are included, as showing maximum stats gained is 30, so even one good night will bump you significantly.
Assuming everyday training, every ten days getting a stat boost a horse should get about 12 stat boosts in it's lifetime. which will be 11-21 stats each depending on the horses' level, Starting at 11 for Novice and going up 2 stats each gained grade level.

I think the foal stats is something like Mare+Stallion/4 with some randomness and discipline bonuses thrown in so assuming you get your first generation to 800 stats the second generation should start out somewhere near 300 stats, thus moving your training boosts up almost immediately to starting at Local level (13 stats) so instead of about 152 stats they should be getting closer to 180-200 stats from training. 
Assuming the same average 4 stats a day (1 from treats 3 from RS or showing) Any horse will gain at least 500 stats  from that, plus the 200ish from training ans starting at 300 stats total the second generation should top out at at least 1,000 stats.

Here are two second generations I found in my herd from horses I acquired at about 5 years old from another breeder. They have already surpassed their parents, and still have 7 years of life yet. They are also shown and not riding schooled.
http://www.equiverse.com/horse.php?id=887417
http://www.equiverse.com/horse.php?id=887419

And looking for a third gen in the DWBS to see the variance, here's a second gen that started out much better and has reached over 1300 stats at 18 years:
http://www.equiverse.com/horse.php?id=875544

Basically the better off you start the higher your next generation will be. 

This has been a fun mathematical experiment. 
Please ask if you need clarifications on any point or want more info. 

Thank you so much for this comprehensive explanation, it helped a lot :)