select max(n) + 1 as letra_sig from (select
		case when substring(t.nro_ht from 11) = '' or substring(t.nro_ht from 11)  similar to '[A-Z]'  then 0
		when substring(t.nro_ht from 11) is not null or substring(t.nro_ht from 11) not similar to '[A-Z]' then substring(t.nro_ht from 11)::int end
		as n
from tbl_hojatramite t where substring(t.nro_ht,1,9) ='201945098' order by n) as tbl_copias