What is the minimum amount?

There is no minimum amount when buying bitcoin with Pocket.

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

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