I’ve spent some time mulling over this, and the answer is fairly straightforward.
How many can provide helping dice? As many as can provide a reason to provide to make a relative difference.
In other words; If a player can describe the helping character aiding the task in a way which makes a relevant difference. He can help.
If (s)he cannot, he still participates. But does not add any dice to the total pool.
A group of characters are attempting to sneak into a religious ghetto. Security consists only of human (ie:failable) guards with only simple helping gear. (Light amplification units) And is essentially Low.
This calls for a Stealthy check (or is it Recon in BE? I forget)
GM being fiendish tasks the Ironlord socialite (wigged and all) to lead. (being the weakest link)
There are 4 characters all together. The mentioned Ironlord, his Kerrn bodyguard, an annointe Priest of the wheel (cotar?), and the self-sufficient commoner (a rising star) the one who originally found evidence of the valyen infiltration.
Ironlord: 4 Agi
Kerrn: 5 Stealthy: Described as scouting ahead of the team. +2 Dice
Priest: Despite having stealthy opened (former military) he fails to describe a relevant difference. ie: not helping. He’s also on the look out.
Idealistic farmboy: 3 Stealthy. Described as sighing and helping the courtier strap his finery proper. Relevant difference. +1 Dice.
This prevents the endless helpdice stacking.
3 Ships in space making an observation roll.
Unless each of the three ships are being described as ‘scanning’ nominaly (and relevantly) different location. They are simply not making a difference compared to each other.