can I buy a gift card of 10$ from a card of 50$

If you mean buy a physical Apple Gift Card or App Store & iTunes gift card:

That depends on the points of sale where you want to purchase the card.

This card (as an example) has an amount between 15 - 200.

Then you have an option to choose what amount you want to buy.

You will make this known at the points of sale.

The minimum amount on a card depends on each country.

Where to buy a physical Apple Gift Card or App Store & iTunes gift card:

Actually, you can buy an Apple gift card at many places, it depends of course on where you live.

At an Apple Store or an Apple Authorized Reseller in your region certainly, but also at a good bookstore, supermarket and/or drugstore.

Beware of -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support

Hannnm wrote:

can I buy a gift card of 10$ from a card of 50$

NO. Apple (like many retailers) does permit using a gift card balance to buy another gift card. Sorry.

If you want to buy a gift card online, you must have a credit card on file.

Sorry, I see I mistyped. That should say "does not permit..."