How do I find my apple card number after it is approved? I only have an android phone.

You can't. If you only have an Android, you can only use the Apple Card for purchases directly from Apple and you use your Apple ID for that. Also see the following from How to apply for Apple Card - Apple Support

If you apply online and don't add Apple Card to an eligible iPhone or iPad that you own, you can only use it to make eligible purchases online and in-store at Apple.2

footnote 2: If you apply online, use a supported browser. To access and use all Apple Card features, you need to add Apple Card to an eligible iPhone or iPad that you own with the latest version of iOS or iPadOS. Then, you can manage your Apple Card on your device and make purchases anywhere Mastercard is accepted. If you don't add Apple Card to an iPhone or iPad that you own, you can only use your Apple Line of Credit for eligible purchases online and in-store at Apple.