lost half of my phone contacts after update

When you look deeply you will find that Contacts are neither stored on an iPhone nor on SIM. They are synced to an email account that you may have added and enabled Contacts. It could be Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other email client server.

When you enable Contacts in any of the email accounts they are displayed to you through the Contacts App. When you edit these contacts on an iPhone they are ultimately edited on the client-server.

Please see the pic below for a better understanding...

If you are unable to find your contacts which were stored in the email client, then try this --> Restore contacts, calendars, bookmarks, and more using ...

Check if the email account has been removed from the iOS mail app.

Add the email account that stores your contacts in the IOS mail app and enable Contacts.

What is your default Contacts account? If none, please add an email account and enable contacts.

See the pic below

Unlike some Android devices, contacts are neither stored locally on an iPhone nor in SIM. They are synced to an email account that you had Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other

If you have a corporate email account added through a profile on your device, please check with your organization's system administrator.

Updating iOS on your iPhone should not change your personal details. iOS updates primarily focus on improving system performance, introducing new features, fixing bugs, and enhancing security. They are designed to preserve your personal data, settings, and information intact.

During an iOS update, your device's operating system is upgraded, but your personal data, such as contacts, photos, messages, and other information, should remain unchanged. However, it's always a good practice to perform a backup of your device before any major update to ensure that your data is safely stored.

It's important to note that while an iOS update itself should not alter personal details, it's possible that certain features or settings may be adjusted or improved with the update. In such cases, you may need to review and adjust specific settings to match your preferences after the update.

Always make sure to read and understand the update details provided by Apple before proceeding with an iOS update, and follow best practices such as backing up your device to minimize any potential risks or complications.

If the issues you mentioned are indeed caused by iOS, it's likely that numerous users who have upgraded to iOS 16.5 or 16.5.1 are experiencing similar problems. However, in order for the community on this forum to provide helpful responses, it would be beneficial for you to specify the nature of the issue you are encountering. Without this information, it may be difficult for others to address your concern effectively.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.