There are many ways to do something like this, you never specified how you were connecting to the database.
Try
Data1.RecordSource = "SELECT * FROM Your_TABLE WHERE Your_TABLE.customerNumber = " & Text1.Text
Data1.Refresh
Data1.Recordset.MoveLast
Label1.Caption = Data1.Recordset.RecordCount
** Change the Your_TABLE to the name of your table & customerNumber to the name of the field