My keyboard needs to be charged, but my new Mac didn’t come with the cable I apparently need to recharge it

It should have come with a USB / USB-C to Lightning Cable in the box for the keyboard. Like the one you would use for an iPhone or older iPad.

If you don't have it, you can always use one form an iPhone, iPad or iPod you may have lying around.

Last resort, would be to buy one: link-> USB-C to Lightning Cable (1 m) - Apple

It came with the cable, you just either did not see it or misplaced it. You can use any Lightning cable to charge it, for example if you have an iPhone you could use the Lightning cable that cam with it. If you do not have one, then you will need to buy one. Amazon or E-Bay sells them inexpensively.

Happy to help.