Field settings and cardinality

[colorfield]

It is currently designed like this in the D7 version of Serial, but makes me unsure about the model. Should the site builder be able to define the cardinality of the field (should always be 1, no ?) or define the field as not required ?
Also, it can be considered with Make serial IDs optional, so the settings could probably be replaced by a clearer configuration screen for the end user.
See #5

[colorfield]

Moved on Drupal.org issue queue https://www.drupal.org/node/2833498