From what I remember about recent classes over the last 10 years, 28's on individual ACT scores over as many attempts as you are willing to take it are at the BOTTOM of the MIDDLE 50%. I think that makes individual 28's the 25th percentile of appointees. (or whatever the equivalent is for the SAT)
Some appointees get appointed with less than those thanks to notable achievements in other than academic ways while showing they're still likely to be good enough academically with maybe 25's by having pretty high GPA's, class ranking, due to very solid CFA, and leadership, and athletic accomplishments.
Some appointees get in with only decent, but only good enough, leadership, CFA's, athletic accomplishments, thanks to very high ACT's or SAT's, as well as other academic achievements.
Some appointees get appointed for having solid accomplishments, but nothing exceptional, on EVERYTHING.
The Whole Candidate Score accounts for all of this, probably with minimums in each area.
If your high school record is like many appointees..... Very good in everything but not truly exceptional in any of them, 28's (or SAT equivalent) is a good gauge to be thought of as the lower end competitive from what I have seen and heard. Class rank, (weighted and unweighted), GPA, higher level courses, CFA, athletic accomplishments, strong letters of recommendation, and a good bit of other community involvement matter a lot.
And some areas of the country are more competitive for one service academy while a little less so for a different service academy. Most of Virginia for USNA vs Colorado for USAFA from what I have heard as an example.
And you are simultaneously competing within your congressional district and/or state to be the top nominee, while also competing nationally, if you're not the top nominee for your area, to be an appointee from the national pool to round out the class.
After all of that, for 75% of appointees, 28's are the lower end of ACT's. The other 25% were especially special in other ways.