I am trying to built a text box where I can type in the number of records that I want to select. So if I type in "100" the first 100 records should be selected and then will assign it to user. How do I make built this text box where it would select first 100 nodes and then perform the action of assigning it to user.
IIRC you could build this type of an Entity reference field with the help of Entity reference Views Display in Drupal 7 and the Entity Reference View Widget module.
In Drupal 8 you probably need the wonderful Entity Browser that is usually used to manage media files, but no reason not to use it for any other entity references. So you still need the Entity reference field, you just use the Entity Browser as the widget for that field to control how the choices are presented.
You'll probably want a filter above your results to list a certain number of results (100 for example) and then a "Select all" option to select all the results that have been filtered out (table display may come with that already).