I want to install your software on both Mac and Windows computers. Will I need to buy separate licenses for each type of system?

No. Depending on the resource, you may be limited on the number of computers you may unlock. (It will never be less than two and is often as many as you want.) But in any event, it doesn't matter if these codes are used to unlock Macs or Windows systems. And you are welcome to mix it up, depending on your available hardware.