I lost half of my contacts , how do I restore them ?

See the articles If you can't back up to iCloud - Apple Support

About missing information after you restore your iPhone, iPad, or iPod touch with iCloud Backup - Apple Support

When Calendar and Contact information restores from iCloud Backup, your settings and old data restore first. Then, iCloud pushes your updated Calendar and Contact data to your device, overwriting the old data. This update happens automatically and usually occurs before you notice the old data.

See a thread Deleted contacts - Apple Community

Note : the half of deleted contacts from iPhone can never be recovered , the rest of contacts that are synced with iCloud will always stay .