I agree with Stalker0 {not too surprising there!} that the actual number is really a side-issue on this. Rather the appropriate level and size matters.
The way I have been doing the conversion is to up-level a critter, add an aura, modify the attacks to be simpler, and usually add a 'death burst' effect that leaves minions in the wake of the Throng.
Something else on the other thread I like is the idea of bloodied transformations, I have the following written down, not sure who posted them to begin with
[sblock]
When bloodied, reduce the size of the swarm and possibly add an
effect. Here are som more ideas on effects on swarm units upon
becoming bloodied. Think of these as part of the unit breaking up,
disbanding or plain and simple being killed.
(Brute) Berserk Formation - Decrease the space of the swarm two steps
if Gargantuan and one step if not - e.g. 6x6 becomes 3x3, 3x3 becomes
2x2. The swarm gains an aura 2 (or 3 if it started put as Gargantuan),
creatures that enter the aura or start their turn there take X damage.
(Soldier) Phalanx Formation - Decrease the space of the swarm two
steps if Gargantuan or one step if not - e.g. 6x6 becomes 3x3, 3x3
becomes 2x2. The swarm splits into two identical swarms (overlapping
each others spaces after the split, if necessary). The swarms share
hit points but have separate actions.
(Skirmisher) Guerilla Formation - Decrease the space of the swarm two
steps if Gargantuan or one step if not - e.g. 6x6 becomes 3x3, 3x3
becomes 2x2. The swarm gains a +4 bonus to (all its) speed(s). Also,
the swarm can shift 2 squares as a move action (or an additional
square if a power already grants them a similar ability).
(Artillery) Orb Formation - Decrease the space of the swarm two steps
if Gargantuan or one step if not - e.g. 6x6 becomes 3x3, 3x3 becomes
2x2. The swarm cannot be subject to unvoluntary forced movement. Also,
the swarm no longer provoke opportunity attacks when using ranged or
area powers.
(Lurker) Scattered Formation - The swarm splits into a number of
minions. The number is twice the amount of single creatures capable of
fitting along a single side of the swarm (e.g. 2x2 swarm becomes 4 1x1
swarms of medium creatures, 4x4 swarms becomes 8 1x1 swarms of medium
creatures or 4 2x2 swarms of large creatures). The minions are placed
anywhere within the space formerly occupied by the swarm, and can
shift (or teleport, if it is appropriate) their speed as a free action
upon being placed.
(Controller) Controlled Formation - Decrease the space of the swarm
two steps if Gargantuan or one step if not - e.g. 6x6 becomes 3x3, 3x3
becomes 2x2. The swarm can alter the space it occupies by one step
once per round as a free action, reverting to its real space at the
start of its next turn. Also, the size of its bursts and blasts are
increased by 1.
[/sblock]
In play this has worked very well and the players felt like they were wading through hundred of enemy Soldiers. I was clear to introduce the Throng in a meta-game way stating "around the corner come 50 Ragesian Veteran Warriors, the kind you faced back in the battle of the Obelisk. I am now treating them as two 'monsters' rather than 50 individuals. What this means is that you will face over 300 enemy combatants in this encounter!"
However, I am not sure that I will be able to scale up beyond this level well and the capstone battle is at 30th level. Meaning these Huge sized platoons will be minions and the PCs will face, literally, thousands of enemy combatants.
I do like how the XPV version tracks well with the system I already use for up-leveling monsters into the higher tiers by changing their type from Solo to Elite to Standard to Demi-monster to Minion. Changing to a Throng allows me to continue to use that Ragesian Master Archer that the group faced way back at 4th level
