Can I combine kitchen exhaust and toilet exhaust for residential apartments of a multi story building. Is ther any code giving any ASHRAE code giving these regulations.

Only if negative pressure is maintained in the system at all times, to prevent exhaust from moving from space to space.

I did not know you could combine kitchen (greasy) with bath fan at all. But I guess I learned something today.

I presume this is a multi-story building with a full-time mechanical exhaust system. The same would not apply to a residence. Depending on the floor plan layout, the exhaust system might have one fan for all exhaust, if convenient; otherwise, it might have separate fans and ducts for kitchens and baths, rather than having ducts on each floor running all over the place.

You can combine exhaust for multiple baths together, and multiple kitchens together, although you would not ordinarily combine kitchen and bath exhausts because of odor migration. There are specific requirements vis a vis make-up air and for fire dampers. Look at the international Mechanical Code 2006 chapters 5 and 6 for the specific sections that relate to your structure(s).

I agree this. Now my situation is that I have to do a heat recovery system also in this. because this is a 35 storied residential building and if possible I can utilise a big quantity of air for heat recovery.

I assume your taking about a air to air heat recovery system. This will work for a short period of time until the system plugs with the grease from the kitchens. This will be a maintance nightmare. Given the amount of air loss with each unit although combined may seem like a lot once totaled up, simply does not warrant using this type of device. The payback will take years, add to that the service nightmares I do not think this is worth it.
Even if you could install a recovery sytem. Where would you send the replacement air to once the exhaust air heat has been captured? Each apartment? Does not make economic sense.