My most recent broody insisted on nesting in the highest box possible in the coop! Fortunately the 'nest' was in a cardboard box, so the evening that I started to hear peeping inside the eggs I moved the box (hen, eggs and all) down onto the floor, in a corner where I could barricade it off from the rest of the coop for a day or two. I left a shallow dish of chick mash and a bowl of water with marbles in it for the hen (the chicks are able to eat and drink when they are a few hours old, although they can survive without food or water for 2-3 days as they absorb the yolk sac from the egg). The marbles (or small stones) are very important - they leave enough space for the hen to get her beak in between and drink, but mean that the water is not too deep for the chicks. If day old chicks fall into a bowl of water they risk drowning.
With the setup you have in the photo I would suggest you move your broody to a new place / cardboard box with her eggs a day or two before they are due to hatch (move the eggs first, and the broody last) - then you won't risk any chicks falling out of the nest and not being able to get back up.
If you don't get as many chicks as you want then you can always add a few more, but you need to do it within a few days of your own chicks hatching, and you need chicks that are no more than a couple of days old. Chickens are notorious for not being able to count - they don't know how many eggs they sat on, or how many chicks they have, so she won't know that you added extra.
However, if the chicks you add are more than a day or two old then they won't 'imprint' the hen as their mother. This means they won't respond to her calls and follow her around, and she won't be interested in them if they are not making their 'looking for a mommy' call. Week old chicks would be too old - they are no longer looking for a mom, but just getting on with things on their own.
Best of luck!