From a practical point of view, up to two characters can help
From the definition of assets, on numenera discovery, pag. 103
An asset usually eases a task by one step. Assets can never ease a task by more than two steps—any more than two steps from assets don’t count.
From the definition of helping, on numenera discovery, pag. 118 (enphasis mine)
If you use your action to help someone with a task, you ease the task. If you have an inability in a task, your help has no effect. If you use your action to help someone with a task that you are trained or specialized in, the task is eased by two steps
These are the only relevant rules. You may have as as many characters helping you as the situation allows (which means that the GM decides) but there is a limit on how much of that help really has any benefit, and that is either two untrained characters, or a single trained character.