2 units of 30. maybe even three. at least one with something either side that can flank-charge any chargers.
having a 'horde' type army isn't just about taking hordes - it's about taking more units than the opponent, which will guarantee charges for some of your army - there's no way he can charge it all at once!
now, i've not used goblins yet, but as they're akin to skaven in the basic use of them, i'd suggest taking units of 20 in 4 ranks. i commonly run 4 to 6 units of shield-spear and shield-ccw skaven in my list. they do quite well holding their own, but a single unit of weak'uns generally doesn't do well - it only has to lose 1 combat and your outnumbering unit is all gone - the other thing with 2 squads is he can only persue one if you break from combat, so 1 will survive and have a chance to regroup and get back fighting.
as most units are run in either horde (10 wide) or 'normal' (5 wide) ranks, having a horde is generally less advantage than spears - if a 5-wide squad charges you then only 5 or 6 or so of your models will be able to attack at the front, so 3 ranks of 5 or 6, wheras a 5-wide 4-ranked unit with spears can attack about the same amount, for cheaper, and stop hordes with higher initiative from getting every model attacking by being only 5-wide. then you can get 2 units of 20 with spears for a bit more than a unit of 4o without.
but the big thing is to stop thinking of each individual unit as 'did work' or 'didn't work' and start thinking of how it can work in the army. not every unit can hold its own on its own, and treating every unit as individual is a sure way to lose against an opponent commanding his forces as a whole. (think of the difference between me throwing a handful of marbles at you (a bit painful) and throwing a bag of marbles at you (very painful).
hope this helps!