Yes you can stake any amount, the frequency of staking rewards is what will change based on how much you stake. 2.8 PHR is given to the staker who mines each block which is every 60 seconds.

The ROI will depend on how many coins are being staked over time, which can fluctuate and also will go up or down based on how many masternodes are running, because every additional masternode removes 10,000 PHR from the potential pool of staked coins (it has a separate pool of rewards, currently 4.2 PHR per block).

I recently ran through the calculator we have pinned to the #staking channel in Discord and on the website, and if I assume that all possible coins are being staked, removing the current number of masternodes, the annual staking ROI would be about 21%. Of course not all coins will be staked, including potentially those sitting in exchange wallets or in cold storage, so the actual ROI is likely somewhat higher.

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REMINDER: We have a mandatory wallet and masternode upgrade that needs to be done prior to March 9th (block 250,000). Cryptopia has already upgraded their wallets and all other exchanges that trade Phore have been notified. Staking and masternode rewards could be affected if you have not upgraded your wallet or masternode by then. Instructions are available on our GitHub page for the v1.2.2 release for upgrading both wallets and masternodes. Please stop by our Discord if you need technical support.

https://github.com/phoreproject/Phore/releases/tag/v1.2.2