The number of files actually kept in the /var/log/sa directory may be slightly higher than the HISTORY value due to the way the sa2 script figures out which files are to be removed (see below "How the sa2(8) script applies HISTORY value"). Using a value of 28 keeps a whole month's worth of data. If you set HISTORY to a value greater than 28 then you should consider using sadc's option -D to prevent older data files from being overwritten (see sadc(8) manual page). In this latter case data files are named saYYYYMMDD and reports sarYYYYMMDD, where YYYY stands for the current year, MM for the current month and DD for the current day.
How the sa2(8) script applies HISTORY value
The sa2 script uses the find command with the -mtime option to figure out which files are to be removed. The find command interprets this value as "N 24 hour periods", ignoring any fractional part. This means that the last modified time of a given sa[r]DD data or report file, using a HISTORY of 1, has to have been modified at least two days ago before it will be removed. And for a HISTORY of 28 that would mean 29 days ago.
To figure out how a HISTORY of 28 is applied in practice, we need to consider that the sa2 script that issues the find command to remove the old files typically runs just before mid-night on a given system, and since the first record from sadc can also be written to the previous day's data file (thereby moving its modification time up a bit), the sa2 script will leave 30 files untouched. So for a setting of 28, and counting the data file of the current day, there will always be 31 files (or 30 files, depending on the number of days in a month) in the /var/log/sa directory during the majority of a given day. E.g.:
April 30th: 31 files (Apr 30th-1st, Mar 31th)
May 1st: 30 files (May 1st, Apr 30th-2nd)
Yet we can note the following exceptions (as inspected at Noon of the given day):
February 28th: 31 files (Feb 28th-1st, Jan 31st, 30th & 29th)
March 1st: 30 files (Mar 1st, Feb 28th-2nd, Jan 31st & 30th)
March 2nd: 29 files (Mar 1st & 2nd, Feb 28th-3rd, Jan. 31st)
March 3rd: 28 files (Mar 1st-3rd, Feb 28th-4th)
March 4th - March 28th: 28 files
March 29th: 29 files
March 30th: 30 files
March 31st: 31 files
(Determining the number of files in March on a leap year is left as an exercise for the reader).
Things are simpler if you use the sa[r]YYYYMMDD name format. Apply the same logic as above in this case and you will find that there are always HISTORY + 3 files in the /var/log/sa directory during the majority of a given day.