I have a just bought a Ledger Nano S and have put my Bitcoin onto it using one of the keys it generated. It actually generated 19 of them so I used the first one. My question is, if I want to load further crypto onto it, do I use the same key or one of the others? Thanks.
The string of characters you sent your btc to is called a bitcoin address. You should use a new one every time you receive funds.
This is important for your privacy, and the privacy of those you transact with. The blockchains is a public database, if you just used one address repeatedly, then every person that paid you would be effectively able to see your entire financial history. Obviously this is not ideal, but using a new address for each transaction can help a lot with this issue.
There are some small security considerations as well, but they are more theoretical in nature. Privacy is the main reason.
For more info see: https://en.bitcoin.it/wiki/Address_reuse