Questions.. Is that realistic? And, if so, for how many people?
Very unlikely. Look at it this way. You finish school, get a job say paying $65k a year to start. Now you get married, have a few kids. Say by that time you are making $80K. Well, a house is going to cost an easy $1200 a month, throw in car notes, insurance for both and then health insurance. That's another 1800-2k a month. Every day expence for day care or school, that's another 5k a year per kid. Now house expence, throw in around $700. Put all that together after taxes, you don't have shite left.
20% is very high at the start of your life. I wasn't able to put that much in till I finished up with the kids. Before that, I was able to save around 12%.