why can't i put the 21 emojis

If you have not updated your iPhone 12 to iOS 16.4, you don’t have all of the available Emojis.

If you have updated to at least iOS 16.4, you have all available Emojis.

Here is how to: Update your iPhone or iPad - Apple Support

If ypu are having issues updating to 16.4, please post more info about the issue or error.

iOS 16.4.1(a) is current.

Please read the following article then post back with your question –

Writing an effective Apple Support Commun… - Apple Community