If you would like to enable a Unit field on the line items, you can do this in Business Settings > Purchase Orders tab.
Firstly enable the feature and then add the units you wish to show. In the above example we have 13 different choices. There is no limit.
You can then choose whether to display this for all suppliers or specific types of suppliers. If disabled, you can choose which supplier Types to enable the units for.
This requires you to to have edited some of your suppliers with a Type value.