My ipad want allow me to enter my cell number for receiving messages

An iPad, whether a Cellular model or not, is not capable of accessing Cellular Voice or SMS/MMS Messaging features. An iPad with Cellular connectivity alone can only access IP-data services; the SIM phone number simply identifies the associated cellular account for billing purposes.

If paired with an iPhone, the capabilities of iPad significantly broaden to include Cellular Voice and Messaging services of the iPhone - relayed to the iPad over WiFi using Apple’s continuity features.

Continuity

https://support.apple.com/en-gb/HT204681

SMS/MMS Messaging

https://support.apple.com/en-us/HT208386

WiFi Calling

https://support.apple.com/en-gb/HT203032

To use SMS/MMS messaging from the iPad Messages App, your iPhone must be logged-in to iCloud with the same AppleID - and message forwarding to your iPad must be enabled on the iPhone.

https://support.apple.com/en-gb/HT208386

I hope this is helpful in clarifying your iPads Messaging features.