Air flow in rub

So i have a 33l rub and i only put 3 holes in one lenght side in the middle of it.

My humidity is always between 65 and 75

Do i make a couple holes in the opposite side?
Im just worried about air flow, oxygen available and air stagnation.

I would put enough holes to get your humidity at an appropriate level. I use a soldering iron to burn the holes into the tubs - works great.

Yes melt a few holes in the tub with the iron

I would add a few more holes... roughly 1/8"-1/4", so you have some on both sides. If you get too many, you can either cover them from the outside with tape, or just add a larger water bowl to bring the humidity back up. I had a 15qt tub with 30 holes it in. :)

Thanks