If you have a big number of child items and you want to use the parent as source path in a Lookup, Multilink or Valuelookup field you might find that not all items are shown.
The default limit is 100 and is set in web.config and can be changed:
Specifies the max number of items in a query result set.
This also controls the number of items in Lookup, Multilink and Valuelookup fields.
<setting name="Query.MaxItems" value="100" />