How can I enable the Alphanumeric Keyboard on my iPhone 12 Pro?

By default iOS devices do not have Alphanumeric Keyboard 

Be aware that using a third-party keyboard on your iPhone can potentially expose your data to the keyboard developer. Here are some key points to consider:

1. Data Access: When you install and use a third-party keyboard, it can potentially access all the text you type, including sensitive information such as passwords, credit card numbers, and verification codes. This is because the keyboard needs to read your input to provide suggestions and autocorrections.
2. Permissions: When you enable a third-party keyboard, iOS may ask you to grant the keyboard "full access." Full access allows the keyboard to communicate with its developer’s servers for additional functionality, like word suggestions and custom themes. Without full access, the keyboard can only function locally and cannot share data.
3. Trust and Privacy: Trust is crucial when choosing to install a third-party keyboard. Reputable developers typically have privacy policies and measures to protect your data. However, there's always a risk that less scrupulous developers could misuse the data they collect.
4. Apple’s Safeguards: Apple tries to mitigate risks by allowing users to toggle the "Allow Full Access" option and by not allowing third-party keyboards to operate in sensitive areas, such as password fields, by default.

ridwaan23 wrote:

which keypad can I download from iStore

That's the only way to get what you want. I think common apps that users try are swiftkey, bullkey, number row keyboard, fleksy.