Assuming this hood has a filter or baffles, you can perform the following process to obtain what I consider to be adequate flow. Note that this is an extremely shortened description that can be expanded by searching the forum for more hood information.

Measure the hood's entrance aperture dimensions and compute its area in square feet. Multiply by 135 cfm per square foot. This is the rated zero static pressure blower to aim for, assuming a nominal fan curve and nominal pressure loss in the path from outside to outside. Slightly lower will be fine, particularly with such a short duct, but this depends on how tight your house is for supplying make-up air to replace what the blower removes from the kitchen, and how dirty you let your mesh filter (if any) get.

For example, a 2 ft x 3 ft aperture is 6 square feet, deserving an 800 cfm rated blower. (Your actual aperture may be smaller due to structural blockages.)

Due to the nature of capture and containment, the number of operating burners is of secondary importance to required flow velocity at the hood filter or baffles.

kas

You could use square/rectangular duct instead of round to increase your capacity by 30+% in the available space