Index: src/js/media/models/attachments.js
===================================================================
--- src/js/media/models/attachments.js	(revision 51190)
+++ src/js/media/models/attachments.js	(working copy)
@@ -212,6 +212,8 @@
 		this.observers.push( attachments );
 
 		attachments.on( 'add change remove', this._validateHandler, this );
+		attachments.on( 'add', this._addToTotalAttachments, this );
+		attachments.on( 'remove', this._removeFromTotalAttachments, this );
 		attachments.on( 'reset', this._validateAllHandler, this );
 		this.validateAll( attachments );
 		return this;
@@ -237,8 +239,32 @@
 		return this;
 	},
 	/**
+	 * Update total attachment count when items are added to a collection.
+	 *
 	 * @access private
 	 *
+	 * @since 5.8.0
+	 */
+	_removeFromTotalAttachments: function() {
+		if ( this.mirroring ) {
+			this.mirroring.totalAttachments = this.mirroring.totalAttachments - 1;
+		}
+	},
+	/**
+	 * Update total attachment count when items are added to a collection.
+	 *
+	 * @access private
+	 *
+	 * @since 5.8.0
+	 */
+	_addToTotalAttachments: function() {
+		if ( this.mirroring ) {
+			this.mirroring.totalAttachments = this.mirroring.totalAttachments + 1;
+		}
+	},
+	/**
+	 * @access private
+	 *
 	 * @param {wp.media.model.Attachments} attachment
 	 * @param {wp.media.model.Attachments} attachments
 	 * @param {Object} options