Does Defender bonus increase the longer you stay/hold a gym?

No it's still the same. From my experience - I stayed in the gym for one week.

And even only first 10 assigned pokemons to gym are rewarded. I mean: when you have 12 pokemons you'll get the same bonus, you'll get when holding 10 of them (5000 + 100).

As of the new update that was just released, the length of time your Pokémon defends a gym determines the defender bonus entirely.

From this article detailing the new system:

First of all, the defender bonus is now based on the length that your Pokemon stays in the gym, and so the longer that it remains defending a gym, the higher your eventual defense bonus will be; you won’t just receive a set bonus once a day.

But you also do not collect the bonus once every 24 hours anymore. Instead, you only collect it when your Pokemon is no longer defending that gym. Whenever the Pokemon returns to you from the gym, it will bring with it all of the coins that you have collected over that entire period. You still can only collect 100 coins per day maximum, though. You will receive a notification when the Pokemon returns home from the gym informing you how many coins you have collected.