>I am just not sure where they get 10 records? Do you have an idea?
According to the information you show:
Assuming that the general external merge-sort algorithm is used, and that the available space for storing records in each page is 512−12 = 500 bytes, each page can store up to 10 records of 48 bytes each. So, 450 pages are needed in order to store all 4500 records, assuming that a record is not allowed to span more than one page.
Because it is assumed that a record cannot span more than one page, each record is 48 bytes, then for a page (512−12 = 500 bytes), so a page can store at most 500/48=10. The extra 20 bytes is not enough store a record.
In SQL Server, A buffer is an 8-KB page in memory, the same size as a data or index page. Thus, the buffer cache is divided into 8-KB pages.