How many supervisors for a post-doc position?
There are oh-so-many things to govern this decision.

First, a post-doc should be about you establishing your own vein of research and experience in handling a grant. With too many "superiors" involved, you may end up doing little more than answering to them, as a graduate student does. This is not a dissertation committee, it should be a working group of peers that launch your career. From your comment it sounds like there is heavy redundancy in your 'team.' If you are doing an ecological experiment with X population for A behavior and B technique -- which thing is your expertise? It sounds like your working group could 'work' entirely without you. That's not good. 

There are two ways you can approach this. 


Only have supervisors who are more senior experts in your exact area of interest, thus ensuring you get the pedigree of an area expert through this project.
Only have supervisors who are not in your area of interest so that you are the area expert. For example, if you are the behaviorist and are trying to establish yourself in your field as such, don't answer to other behaviorists, only methodologists. Be your own boss.


Choose between strategies 1 and 2 based on your long-term career goals.

There are other, minor considerations: you need to carry at least one person who has a solid track record of competently handling scientific funding to be co-investigator. Also, if you do not have a few publications in the area, you better include someone who does on your grant proposal. It would be ideal if the funded/published is the same person. 

Finally, don't burn bridges. You can always publish with someone later.