How can I create tree like subsub...section?

[khajavi]

How many levels of subsections, docusaurus support?

Can I have subsubsection on sidebars?

[slorber]

Docusaurus 2 support as much levels as you want and your theme can render properly, so it's your responsibility to remain reasonable ;)

    {
      type: 'category',
      label: 'Docusaurus',
      items: [
        {
          type: 'category',
          label: 'Docusaurus',
          items: [
            {
              type: 'category',
              label: 'Docusaurus',
              items: ['introduction', 'design-principles', 'contributing'],
            },
          ],
        },
      ],
    },

[khajavi]

Nice!
Thank you @slorber