If we add 9 odd numbers together we always get an odd number. Assuming the 9 rectangular boxes are of different sizes, we can put a small box into a larger one. Let's put one object into a small box and then place this box into an otherwise empty larger box. Effectively the large box contains one object. We've used two boxes so we have 7 left to fill with 49 objects, which means we could put 7 objects into each of the 7 boxes.
Another solution is to take out two boxes such that one box (the small box) fits inside the other (the large box). In all but the large box we place 5 objects in each. So we have accomodated 8×5=40 objects altogether. We put the remaining 10 objects into the large box and place the small box inside the large box, so the large box now contains 10+5=15 objects.
Here's another solution. Label the boxes A, B, C, D, E, F, G, L, S.
Put 7 objects into boxes A, C, E, G; put 5 objects into boxes B, D, F, S; put 2 objects plus box S into box L (box L now contains 7 objects).
We have 4×7+4×5+2=50 objects in 9 boxes.