Try this

declare @sqlstring nvarchar(1000), @table_name varchar(25), @cntout as int



set @table_name = 'tableA'

set @sqlstring = 'select @cntout = count(*) from ' + @table_name



exec sp_executesql @sqlstring, N'@cntout as int OUTPUT', @cntout = @cntout OUTPUT



PRINT @cntout