If you have a WiFi+Cellular model iPad - and wish to provision Cellular service using a physical SIM card - you will need to obtain a DataSIM suitable for iPad from your chosen Cellular provider. Be aware that many cellular providers also facilitate provisioning using the built-in eSIM.
If you plan to visit overseas, you may wish to consider “roaming” - for which additional charges are usually levied. Alternatively, you might instead use a local DataSIM, or use Apple’s carrier partners for short-term (if potentially expensive) cellular access.
More information can be found here:
Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support
Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support