Is there a 'command' to get the max supply of bitcoin that will ever exist using the bitcoin-cli?
In a sense, such a command would be trusting a third party: any putative answer would be that given by whoever supplied that code to you; and it may not be trustworthy - it could tell you it was 21 million, but behind the scenes actually use, say, 42 million.  

The nearest you could do would be to download the source code from a proper source and, having verified your download, check the file src/amount.h for the line static const CAmount MAX_MONEY = 21000000 * COIN; before building the software yourself. You are now more sure that your node has the proper max supply.