What is the minimum amount?

There is no minimum amount when buying ethereum with Pocket.

Just keep in mind, that buying small amounts of ethereum very often leads to many chunks of ethereum, also called unspent transaction outputs (UTXOs), in your wallet. The more of those little chunks you need to spend your ethereum, the larger the network fees will be for your transactions.

So if you stack ethereum weekly, make sure to consolidate your wallet every couple of months when the ethereum network isn't too busy. Simply do this by making one transaction sending all your available ethereum to a new address of your same wallet.