Considering just the launch aspect of the mission, it depends on how much mass you plan to launch and what launch systems you have availiable.
If there is a vehicle capable of launching the entire payload then it is usually cheaper to launch as one payload (the space shuttle is a major exception, but I won't go into that subject right now ??? ). Multiple rockets would use more fuel because instead of having one large vehicle's support structure and guidance systems, etc. added to the total payload mass, you have multiple support structures, etc. to add into the overall mass (the extra mass of the larger rocket is almost always less than the combined extra masses of the multiple rocket system).
On the other hand, if there is no vehicle capable to launch the total payload (the Proton rocket, one of the largest, is only capable of putting 19,760 kg into Low Earth Orbit at a cost of around $50 million), it is cheaper to use multiple launch vehicles than to design one for this purpose (unless of cousre you are looking at the long time prospects and plan to use this new rocket for other applications than this mission, but lets assume this mission will be unique for the near future).
A third option is also availiable. Hybrid designs using easily designed componets on existing hardware can be a very economical choice, but it depends on exactly what the hybrid consists of. A good example is the Heavy Lift Launch Vehicle (HLLV), which was basically a rocket with the payload put in place of the shuttle on the external fuel tank and solid boosters. It would have been capable of launching around 70,00kg into a Lunar or Martian trajectory! But it never got of the drawing board, and here we are 33 years after Apollo 11 still stuck in Low Earth orbit. I hope I didn't babble too much and I answered your question.
Nuclear Propulsion: The faster cheaper way to Mars!
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