In the path statement you have to put the lower level to get it to look at all the rows.
and then in the With clause indicate text() function to have the Value pulled.
alter Procedure [dbo].[Wty_Warranty_S]
@Warranty_Type_ID nvarchar(2000)
as
Declare @XMLHandle Int
Exec sp_xml_preparedocument @XMLHandle OUTPUT, @Warranty_Type_ID
Select *
from OPENXML(@XMLHandle, 'WTS/WT',2)
With (Warranty_TYpe_ID varchar(20) 'text()') AS Params
Exec sp_xml_removedocument @XMLHandle
GO
Exec Wty_Warranty_S '<WTS>
<WT>2</WT>
<WT>50</WT>
<WT>48</WT>
</WTS>'