Frequent readers of Troll Tales will know that I don't value Night Goblins as highly as other players might do, preferring instead the more sturdy Orc Boyz as the mainstay of my army lists. A lot of O&G players I talked to though, in the very early stages of the new edition, were practically ecstatic about the usefulness of Night Goblins as the ultimate anvil-unit of our army, pointing to the fact that you'll get two gobbos for the prize of one orc, and equipped with nets they'll be almost as tough as an orc and both guys will get the parry save. In fact, for the price of 30 boyz you'll have 50 gobbos with nets which undoubtly means an additional 20 bodies for maintaining steadfast which can only be a good thing. I don't necessarily disagree with that notion (although WS2 instead of WS3 can sometimes cost you) but what I do notice is that too many O&G player, high on the usefulness of gobbo anvil units, tend to overspend on them, which sort of defeats the point of running them in the first place.
Let's make this clear. The main purpose of a big unit of Night Goblins should be to take the charge of an enemy, pass the breaktest (goblins never win on their own) and hold on long enough for your hard-hitters to come help them out, preferably by flank charging the enemy. With that said, there's very few times where 60, or even a 100 gobbos can do what 50 of the little buggers cannot, which is to outrank the enemy to remain steadfast and improve the chances of you holding them in place for a turn or two. In a majority of cases, if 50 gobbos aren't enough then you've either made an error in your deployment or you've too few support units in your army list. I believe a good rule of thumb is that any strategy you employ that depends on your unit losing combat for more than two rounds straight is usually a poor idea. I might even go so far as to put one round as the maximum (don't charge if you don't think you'll win, better to have them charge you then) but there's definitely times when charging something nasty to lock it in place can be necessary. For one or two combats on their own, 50 gobbos should be enough. Paying for more bodies in order to accomplish the same thing is not very efficient.
Which brings me to my next point. This might seem obvious, but too often I notice players forgetting about a very basic rule of thumb. Don't bring an anvil without a hammer. Certainly, the A&H strategy is not the only way to play O&G, but it's a basic strategy that even though enemies might see it coming, it can be surprisingly hard to stop. It's hard to argue against the logic that one of the benefits of having a larger unit count than the enemy is that you'll be able to fight some (if not all) of your battles 2-on-1. With that said, you're going to have to keep in mind how many enemy units you're likely to face where double-teaming will be necessary. I usually feel that 3 anvil units are enough at 2500 points, as there are few armies out there that can field more than 3 nasty units that I won't be able to take on by support charging it with a chariot (quality costs, as they say). At times I've felt that a 4th anvil would have been useful, but I cannot imagine a matchup where 6 or even more large anvil blocks of 30 orc boyz, or 50 night goblins each would be required to do what they're meant to do, which is to take a charge and hold the enemy in place. Unfortunately, too many O&G list builders I come across have failed to understand this, focusing too much on big blocks of gobbos and too little on hard hitting units to support them. Flank charging an enemy engaged with one anvil block, with a second anvil block, will still only provide +1 combat resolution for the flank, possibly one for the charge, and 10 WS2 S3 attacks. Not very impressive, when for a fourth of the price you could instead be flanking with a wolf chariot whose impact hits will probably do more hurt, or if rankbreaking is required, a Squig Herd unit for half the price of a gobbo block will kill thrice as many enemies.
To summarize, the idea of running big blocks of Night Goblins should be to get equally (if not more) efficient anvil units for a cheaper price than Orc Boyz, leaving more points to be spent on support units and characters, which leads to a more cost efficient army. The idea should not be to spend an equal amount of points on double the amount of anvil units, as it's very unlikely that you'll make use of all of them and you won't have the advantage of more points in support units, meaning your army won't work together as well as it should be.
When building an army, consider what roles are required in your strategy, and try to think of ways to complement one role with another one. This is particularly true when it comes to Orcs & Goblins, as practically none of our units will have what it takes to defeat an enemy on their own. This is nowhere as evident as with Night Goblins, who'll need plenty of support in order to win battles for you.