A topic that comes up periodically is that of the use of an assistant first horn in orchestral playing in the United States. I have some general notes here, but a side issue is that of how sections run when they do and don’t have an official Associate Principal horn.
As such, this post borders on materials also discussed in my Orchestra 101 series as we get to the topic of how horn sections actually run. Which in this case on a professional level will certainly vary orchestra to orchestra and master agreement to master agreement.
For example, for a heavy program performed by a top level orchestra with a designated Associate Principal horn the Associate would play principal on the first half and the Principal first on the second, with use of an assistant first horn possible/probable on either half. When I played in Nashville however I was Third horn we had no designated Associate so we would have done the seatings differently on the same heavy program; the Principal would have played Principal for the entire concert but with an Assistant. Only if she was completely gone I moved up to Principal from Third (and received overscale for the services impacted), the assistant moved to third, and an extra was hired to play assistant. Shortly after I left Nashville the third horn position was changed to Third/Associate, so I assume at that time the situation became more flexible and no overscale was involved if the third moved to first for a work or two.
Back to the generic heavy program example, no professional orchestra I have ever played with as a member or extra would require a principal horn to not use an assistant on a demanding program, but I suspect that the times they are a changing in the United States! Management will try to save whenever possible. It makes it really hard to play your best when there is no assistant available, especially if it is a heavy program with just four players, but it does save money and the audience may not notice the difference either, even if your chops do.
I should mention there is another model is that I am told is seen in Europe. They don’t use assistant players but “platoon” larger numbers of horns, with a full section on one half of the concert and another full section on the other half, with perhaps no players performing the full program.
In short, what you experience out in the real world of music will vary but in the best orchestras in the United States we typically see five or six member sections that will function along the lines described above and it is a good model to try to follow if possible in your situation be it amateur or professional.