You would have paid part of that 21 million to miner fees, and there will never be 21 million bitcoin anyway, so not that much.

However, assuming you have a channel directly with who you're trying to pay, you can pay the full size of the channel. If there are hops that will be a very difficult thing to calculate because I would have to know the channel sizes of the nodes routing your payment, the fees they have, assuming a multi-part payment, what sizes all channels along all paths have to contribute and all of their fees.

Channel capacity and liquidity are the limitations.

If you want to send 1 BTC and all channels are 0.5 BTC you might struggle.

There is a concept called multi path payments but that is still in development AFAIK.

It's all about the recipient's wallet type. Before multipath payments, we were usually stuck to something like 1/10th to half a bitcoin, max, on most routes, but technically up to 10 BTC on routes that we knew were going thru someone with a ton of liquidity and no one else. (Rare)

Nowadays we can stack quite a few routes at once so 10BTC wouldn't be a problem on most routes. Still, the receiver's end can cause a bottleneck because they often have hosted wallets that open small channels for them. Wallet of Satoshi, for example, won't open a channel with 10BTC in it for every customer for obvious reasons. Even if you run your own node, do all of your channels have 10BTC stacked together to receive that much in? It's pretty uncommon for all but the largest companies with fat channels to spare.

So the bottom line is to ask your recipient if they have the channels to handle the amount you want to send them. If you're sending it to yourself for some reason, you have to set up the channels first and that can take a while to find that many channel partners.

People used to publish statistics on this sometimes, but I haven’t been able to find any for a while. We really should know what the payment limits are on LN, or at least what the failure rates are for higher amounts.