select @@servername as 'instance name', count(*) as 'Nº of DBs by Instance', sum(a.size) as 'espaço alocado (mb)', sum(a.used) as 'espaço utilizado (mb)', sum(a.size)-sum(a.used) as 'espaço livre (mb)'
from db_name..tbl_name a, sys.sysdatabases b
Your count is high because your are joining with whatever db_name..tbl_name is producing 7921 records.
quick fix, run two queries:
select @@servername as 'instance name', (select count(name) from sys.sysdatabases) as 'Nº of DBs by Instance', sum(a.size) as 'espaço alocado (mb)', sum(a.used) as 'espaço utilizado (mb)', sum(a.size)-sum(a.used) as 'espaço livre (mb)'
from db_name..tbl_name a, sys.sysdatabases b