Adding even one person to the team with the right experience makes the challenge 50% easier but this depends entirely on the challenge. Say for example, if one challenge requires extremely specialized knowledge (c++, face animation, hand painting textures, creating music, photography, math, procedural generation) then it is highly unlikely one person does more than one of the specialized skills. Of course some people do but it is rare. Estimate an average of what size of skillset is reasonable for a student or an expert (4 > 10 years experience) and see if a challenge fits 1 or 10 people.
Edit* Applying by skillset might just be a solution. If instead of “VFX” challenge (which is very very broad) you make a challenge specifically for a skillset (“Particles” + “Texturing” + “Materials” + “programming / math”) You can let people apply for roles in this skillset. One person might take the 4 roles, or 2 people take 2 roles. I see how this would work for students currently studying one or more roles in particular. You can still give the challenge a theme, for example in the above skillset a fireworks show would be great.