What type of SIM card should I buy for my iPad 9th generation

The majority of mobile phone SIMs are not correctly provisioned for use with tablet computers. If they work at all, they may not provide access to the full range of cellular services. This is not a limitation of the iPad, but of the Cellular network provider.

If using a physical SIM with iPad, you will require a DataSIM explicitly provisioned for iPad or mobile broadband (a.k.a. MiFi).

Be aware that an iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. If you have added a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.

Data only SIM, not a SIM for a cellphone.

Ask the provider whose SIM you are contemplating

if that is the correct one for an iPad.

Yup,

The ONLY SIM card you can get for any iPad with cellular capability is a data ONLY sim card and to activate it, an additional cost is, usually required/involved.