When does building extra buildings start to give diminishing returns?

There is a difference in buildings that produce resources (or positively influence resource income) and buildings that reduces the consumption.

In the first case, build whatever you want. They will always add up to your total income. These buildings are for example catnip fields, aquaducts and temples (culture). Every catnip field you build extra will produce another 0.125 catnip per tick, excluding all bonuses. The same goes for the mine. Each mine will increase the mineral output by 20%. Note that this 20% is summed over, not multiplied. So 10 mines increase output by 200%, not 1.2^10 = 619%.

The second part is a bit more tricky. You already noticed that 25 tradeposts should result in a stop of consumption of the luxury goods (because 25 * -4% = -100%). That is indeed not the case. There is a diminishing function, explained here, which will make sure that the reduction never reaches 100%. However, the diminishing does only start after 75%. So building 18 tradeposts (=72% reduction) gives the expected reduction, but 19 tradeposts will give around 75.8% something. Each tradepost you will build after this 19th one will increase the reduction by a smaller amount, as seen in the previously mentioned answer. Pastures (both normal and unicorns) are good examples (and tradeposts of course) of buildings who have this diminishing effect. An example of a diminished effect is the auto-observe chance for observatories.

This diminishing function is the same for every building and effect. So every building and effect increases linearly untill 75%, and after that it's slope will decrease (as shown in the figure in the linked answer).