select top 20
    ld.Artigo
    ,count(ld.Artigo)
    ,sum(ld.TotalIliquido)
from
    dbo.CabecDoc        as cd
inner join dbo.Clientes as c
    on c.Cliente=cd.Entidade
inner join
    dbo.LinhasDoc       as ld
        on ld.IdCabecDoc = cd.Id
inner join
    dbo.DocumentosVenda as dv
        on dv.Documento = cd.TipoDoc
inner join
    dbo.Artigo          as a
        on a.Artigo = ld.Artigo
inner join dbo.Seccoes as s
    on s.Seccao=c.CDU_Seccao
where
    dv.TipoDocumento >= 4
    and cd.TipoEntidade='C'
group by ld.Artigo
order by count(ld.Artigo) desc