MyBB Community Forums

Hello,
As each thread page has 10 posts, then you move to the next page, How do I show more than 10 posts in one thread? (e.g: 20 posts in one thread page)

It's a UserCP preference.

UserCP / Edit Options / Forum Display Options

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Also, default options are set in
AdminCP / Configuration / Forum Display Options / Threads per Page

but this is overridden by different UserCP preference.

as admin you can set via ACP

ACP --> Configuration --> Show Thread Options --> Posts per Page

your users can set via UserCP; possible options you can set options on the page: User Selectable Posts Per Page

edit: HLFAdmin was faster