What does it mean when there's a certain amount of a CW in the cash shop left?

Means only that amount is available for purchase! When it hits 0, the item becomes unavailable; after a month, the creator can re-add that item to the CS, but they may choose to keep it retired as well (so buy while you can) c:

Oh, thank you! There's a few of those items in there that I want, thank you for telling me about that! c:

No problem! Hope you can snag everything you want soon ^_^