Ah well some on us amazon too :)
The single holders would probably work too to give you more flexibility in how you place them, just cause so many cards you won’t need a lot of the time
TLDR; Buy some used Kindles, farm for a few months, flip them out. Rinse and repeat.
You are going to be really disappointed. It is much harder to scale things up than it used to be. There are lots of variables, bandwidth, network, interference. Scaling is not entirely linear. 20 phones will not earn twice as much as ten phones will. In reality double the phones might only have a 1.8x increase in earnings. You'll also need either expensive powered USB hubs or lots of power strips and messy cable problems. Phones will also die, and they will die often. I have a single device that can still function after two years of farming. A single one, out of maybe 50 phones purchased. Most died or had to be sold due to the OS no longer being supported. Average lifespan is about 5-6 months.
The time to spend $500 on a massive farm was two years ago. The best devices, running the most stable apps, will struggle to break $1/day now. Two years ago, I could make $1/day running Perk on a free Kyocera Hydro! Hell, back then you could get a $35 rebate on a $10 phone. They literally paid you to buy those things. Now, you can't do $1/day with Perk on a brand new Galaxy S7.
If you deploy lots of ToS violating automation and multiple accounts, sure you might do alright. But your earnings will be variable and you will constantly need to stay one step ahead of the fraud department. Multiple IP's, if you have access to them, is probably the easiest way to boost earnings.
If you buy Kindles, get them from Amazon Warehouse deals. These are $26-27. You can flip them out on eBay/Craigslist/Facebook for around $30 too. This ensures you don't really have to worry about depreciation. There will always be a market for a $30 kids tablet. If you must spend the money, ten Kindles is the basis for an excellent farm. Mine run RTV pretty well lately. I run Perk Jetpack Journey on them once I start hitting the RTV caps. JJ still only earns like $0.50/day, and that is by far the most stable Perk app on them. They will also run Checkpoints really well. Checkpoints is making a comeback, albeit slowly. However, even ten Kindles might struggle to clear $8-10/day.
You need Google Play Services and Play Store to run some apps on a Kindle. Google "Greggles TV Kindle" I can't really say that my $10 more expensive HD6 is much better than the standard Fire 7, so stick with the 7
As far as phones, diversity is key. Beermoney apps regularly stop working out on certain devices. Sometimes hardware doesn't matter. Seemingly identical devices can earn at wildly different rates due to minor changes in the ROM for instance. I just ordered some $20 Android 6.0 phones, here is how I did it. Here are some more suggestions. A good mix of $20-$30 Android 5.1-6.0 phones will ensure you have devices that can run apps without worrying about your farm being obsolete.
On a closing note, if you have $500 to so casually blow, why farm? This isn't what it used to be, and it likely will never be. This is not a get rich quick scheme, it takes time a dedication to do 40+ devices. At best it's a hobby, at worst a downright chore. Earnings on most apps are really bad now, and I don't see them getting better. You might make a bundle during the holiday ad rush. However, come January, everything grinds to a halt, I'd hate to have so much money tied up. I've been doing this nearly 3 years, it has never really gotten better since it got big in the summer of 2014, earning have always trended on a downward path. At this point my farm is more of an inventory management tool. I get devices for cheap using various methods, and then flip them out after a few weeks. Much more profitable this way.
I used a business card holder to manage my devices for some time. Now I just set them down on a $7 little table from WalMart or on my mini fridge. Double sided tape is a good idea to hold them in place and to tape down cable. Rubbing alcohol can clean up the goo when you need to sell them.
Just get business card holder. I use something like this and just pass it around during strategy phase.
Get some of these guys to hold the cards (works for Eldritch Horror too): https://www.amazon.com/Source-Desktop-Acrylic-Business-BC-8P/dp/B002YTO3JE
Here's the link.
They are great space savers.
I have two of these:
Would these work? Haven't played the game yet so not sure how big the cards are.
http://www.amazon.com/gp/product/B002YTO3JE/ref=s9_hps_bw_g229_i2
Closest I can think of is business card stands. http://smile.amazon.com/Source-Desktop-Acrylic-Business-BC-8P/dp/B002YTO3JE/ref=sr_1_1?s=office-products&ie=UTF8&qid=1442702428&sr=1-1&keywords=multiple+business+card+holder
My table is big enough to fit everything when expanded, but if it wasn't, I'd use these
I have a nice big antique table. There is only one game that I can't fit on it so far - 7 Ages with its 33x45 world map, 15 empire token sets, and up to 7 empire cards laid out in front of players.
However, I do do some tricks to speed setup which also can be considered space savers. First, I have a small rounded-bottom plastic container for my tokens with snap lids - this lets me keep those all together in one neat space. Second, I mix all the location decks for the main board into one big deck. I scan to the color card I need, and if none are left I shuffle the entire discard pile and place it on the bottom of the deck.
However, I also have two other items that might be of use for you: I use an 8 slot business card rack and clear acrylic sign holders. Business card racks can hold decks of cards and discard piles vertically (you discard to the bottom of the decks face up or in a seperate slot), while sign holders can vertically store the ancient one cards and other cards. If your table is really small, get TV trays to set up player stuff on.
I never did do my smaller board redesign, but I find it to be a horrible waste of space. Slightly larger and you could put the cards on the board, but otherwise it could have been a much smaller board and still fit everything on it. Really, it is the only part of EH that I find is a bad design.