Removing streams in HEVC H.265 MKV file?
In the syntax -map 1:a:24, 1 stands for the input file index. Counting starts from 0, so 1 refers to the 2nd input file but the command only has one input. a stands for stream type, and in this case, it's audio - v is for video, s for subtitles, d for data. 24 represents the 25th (0-indexed, remember) audio track. If your command has one input file with three video, three audio and three subtitle streams in the following order: V A S A V S V A S, then the 2nd video will be -map 0:v:1 You can reference it using absolute index by using -map 0:4. With absolute indexing, there's no stream type specifier.
Assuming the stream numbering in your description is correct, use
ffmpeg -i IN.MKV -map 0:0 -map 0:a:0 -map 0:a:2 -map 0:s:23 -map 0:s:24 -c copy OUT.MKV

Note that if -c copy is omitted, all streams will be re-encoded - not what you want normally.