If the post is auto-post eligible, up to 20 tagged Instagram profiles can be added to the post.
Only Public Instagram Profiles can be tagged (private, incorrect Instagram Profiles will not be tagged)
Tagged Instagram Profiles will not be tagged for posts directly published to Facebook
User Tagging Option
User Tagging Editor | Tap Image to Tag
Click anywhere on the image to start tagging Instagram Profiles
User Tagging Editor | Click Done to Save
Click DONE to add tagged users to the post