Follow the example here to create a section properties model and assign it to your section.
Then you can add a dropdown property to the section like this:
public const string ColumnsDataSource = "1;1\r\n2;2\r\n3;3";
[EditingComponent(DropDownComponent.IDENTIFIER, Order = 1, Label = "Number of Columns")]
[EditingComponentProperty(nameof(DropDownProperties.DataSource), ColumnsDataSource)]
public string Columns { get; set; } = "1";