How to set reference frame to 1 while using ffmpeg with h264_videotoolbox encoder

Question

ffmpeg verison 6.0.1 codec: "h264_videotoolbox"

I tried to set codec context's ref to 1, but it's not working. also I tried to set gop_size to 2, same result. An iPhone X encode h264 video c->ref always 7. what happened there, how could I specific reference frame number.