I'd count it as 1 step in a multistep process. Each step of the process gives more info, but is also more investment from each side.
I've also never seen it split over multiple days. Often, pre covid, the company is flying people out and paying for hotel rooms and food at this stage and the interviewee is taking time off from their current job. An extra day adds unnecessary costs to both sides. If I was interviewing somewhere and they tried to split up this step I'd probably get annoyed and potentially cancel the interviewing process.
Counting it as 1 step doesn't communicate what it's like.
You could theoretically have ten 1 hour interviews, back to back in 1 day, and call it 1 step in a multistep process, but most people would call it excessive.
I've also never seen it split over multiple days. Often, pre covid, the company is flying people out and paying for hotel rooms and food at this stage and the interviewee is taking time off from their current job. An extra day adds unnecessary costs to both sides. If I was interviewing somewhere and they tried to split up this step I'd probably get annoyed and potentially cancel the interviewing process.