So what you're asking is what to expect when CPU mining on approx 20 Raspberry Pis that you have spare. I can help you with some numbers as I've demo'd it on a Raspberry Pi 3.
One Raspberry Pi 3 can achieve 7.5H/s per CPU thread. (The raspberry Pi 3 has four available.)
For stability and so as not to over-temp the Raspberry Pi I used 3 threads in my test, and was pool mining using lucasjones cpu-multi miner and sending solutions to http://minexmr.com/ pool.
The Pool reported a hashrate of 25H/s for the one Raspberry Pi using three threads.
Assuming $0.10 for electricity the one Pi would yield 0.01XMR per month, costing $0.72 for electicity. And an estimated profit yearly of $15.02 at current difficulty and price.
This data is all taken from https://www.monero.how/monero-mining-calculator using the Pooled mining calcualtor. The Pi consumes 10W of power, (5v x 2A).
So for 20 Raspberry Pis expect 0.11XMR per month, an extra $14.40 for electricity and 300.44USD per year profit.
The Pool http://minexmr.com/ auto pays out at 0.5XMR, but manual payments can be requested at any point but incur a 0.004XMR fee. Pool also charges 1% usage fee.
Hope that answers your question. For Monero anyway.
It cannot be stated enough though that any long term estimates on price/profit and difficulty over 4 weeks into the future are wildly inaccurate. Don't mine this for an investment, do it as a hobby. Raspberry Pi's are not well suited to this task. But of all the currencies Monero is one of the better suited to CPU mining.
If you want to invest, probably best sell the Raspberry Pi's and buy Monero with the money.
Edit: For your convienience/give it a try...
This runs a script to install the miner and configure it.
On the Pi:
sudo apt-get install git
sudo git clone https://github.com/shingolavine/raspberrypimining.git
./minerd -a cryptonight -o stratum+tcp://pool.address.xxx:port -u YOUR_WALLET_ADDRESS -p x
Those commands will install the lucasjones cpu-multi miner and start mining with 3 threads as default. It is noted and is a known bug by users that hash-rate will display as 0 until a valid share is found.