There's no separate quantity, size, or color field on a wish. Put those details in the wish's Notes or Description instead, so gifters know exactly which version you want.
Steps
Open the wishlist and select the wish.
Add the specifics to the Description or Notes — for example, "size M, navy blue, two of them."
Save your changes.
Keep in mind
Be exact about the variant. If the product page shows several options, name the one you want so there's no guessing — for example, "the 500 ml bottle, not the travel size."
A wishlist isn't a cart. Saying "two of them" in the notes is just a request to your gifters; Wishpicks doesn't sell or count items, and people still buy from the original store.
Notes and description are both visible to gifters, and that's the point — use them to spell out what a link alone can't say. See "What should I write in a wish description?" It's worth setting a priority too, so gifters know which wishes matter most; see "How do I add notes, priority, or details to a wish?"
You need Contributor access or higher to edit a wish on a shared wishlist. See "What is the difference between viewing and editing access?" in the Shared Wishlists collection.