How designers decide oven zone length, no. of zones required for given biscuit variant?
It very much depends on the product type(s) you plan to make and the throughput capacity you need to match your other equipment , as Aurangzeb says. Also the degree of flexibility you need is important. If for example you were going to make only Cracker type products you would want DGF with high performance burners for the early stages of baking and ideally a pre-heat section on the return circuit. On the other hand if you were making only cookies you would probably opt for a convection oven with the benefits of more even baking. Hybrid ovens , where the first half of the oven is DGF and the second half is convection offer the most flexibility if you have a range of different product types to make.
In any case all good oven manufactures will calculate the exact thermal requirements based on available cost, available footprint for the oven and dough making and wrapping capability (either existing or planned).
Hope that helps