On the question of fish and tank size...there are several factors to consider. Species, numbers (for a shoaling species), activity level of the species, water parameters, aquascape, tank size, live plants. Each of these factors affects the tank's biology and thus how many fish or which fish species, because they each affect the fish's biological response. Fish body mass to water volume is a factor but not the only factor of importance, as all of the other factors must be brought into the equation.
Talking pygmy cories. Assuming all else being equal, a group of 9-10 pygmy cories will have less impact on the system than will a group of 4-5. This assumes a reasonably sized tank to begin with. And a 30 liter tank with the dimensions previously given is reasonable. A group of 9-10 of a Boraras species can be housed in this tank just as well.
The dimensions given are 40 X 26 X 34cm. This equates to 16 X 10 X 13 inches. Volume is 9 US gallons, or 34 liters. This tank is only 2 inches shorter in length than a standard 10 gallon. My 10g has had 20 pygmy cories (parents and many fry) and 11 Boraras brigittae. No filter, no light, just a heater. Obviously planted. I'll attach a photo. The July 1-10 photo is the tank in the window, where it was an experiment using natural light and no filter; it ran like this for a year before I moved it, added a sponge filter and light, and same fish. Second photo is today; I've moved the Boraras in with my Chocolate Gourami, and added some oak leaves for the continually spawning cories, and also my Farlowella vitatta fry that grow out in this tank. Other photos show some of the cories, female with fry from two separate spawnings sitting on a leaf. And one of a group of five cories and a couple Farlowella fry on leaves.
The volume is not as important as the dimensions (length and width), and the OP's tank is only 2 inches shorter, and same width, as my 10g. There is more than ample space for what I have suggested, and more beyond that if one carefully selects the species. Even without any filter, this ran well for a year.