How can that be (Minecraft)?

I looked for and found a new island in my Minecraft world.

It's a small island and there are many other small islands around it.

When it's night there are around 20 monsters on my island.

I play minecraft on Easy difficulty in survival mode.

Is that normal? On my main island, I also have to fight many monsters at night.

Yes, it is normal. Less spawn area = higher spawn density.