When approve unless trust level and approve new topics unless trust level are set to 0; Trust Level 0 (TL0), then any post and any topic any TL0 user makes must be approved.
I can’t immediately see a way to allow you set it so you only need to approve one post, as it is baked into our Trust Level system to help new members participate and earn trust before contributing fully and it is something I wouldn’t recommend adjusting.
I do not in any way recommend the hack I mention below, as we always advice not to reduce the Trust Level requirements so you don’t inadvertently harm your community, please take them with a grain of salt.
A hack around this though could be you reducing what a user needs to get to Trust Level 1 (TL1). You can do this by adjusting the tl1 requires ... site settings so users get to TL1 faster and no longer need their posts approved.