I can't speak for everyone and I definitely don't have 20 gpu's (more like 6).
I consider zcash my savings account, I put as much as I can in when I can. Any overtime I work goes into more parts, part of my paychecks go towards more parts, I made a few deals such as let me borrow $600 and I'll give you back $800. Then I pay them back $400 a month out of my paycheck while everything the cards make don't get touched.
I also invest and trade with other alt coins when I see a way to make a quick profit, then back to bitcoin and either to zcash if I suspect the price to go up or back to fiat and into more parts.
Then there's bitcoin gambling but I won't get into that as that's not the best way to go about it as the house edge bites hard.
I suspect people with 20 gpu's either acquired them from mining other currencies over a period of time or are fully committed and is investing whatever they can. You can reinvest your earnings into more parts as well. Say your right makes $650 a month, you buy a new 1080 ti every month with your earnings, now you make more then before. You do it until you double your monthly income then you pay yourself back.
It's hard to say and I doubt people will be jumping over the idea of telling you how they got to where they are. Too many variables