Numbers 1-6: 9 each (54). There is no combination of 4 non-repeating hymnal numbers that would require 10. This part was correct.

Number 7: 8 plates. Remember, there only 700 hymns, so there would be no hymn 777. The most you could get is eight, such as 677, 577, 477, 377.

Numbers 8 & 9: 8 each (16). Since there are no hymns above 700, you’ll need no 8 or 9 in the hundreds place. 688, 588, 488, 388, eg.

Number 0: 8 plates. There’s no need for a 0 in the hundreds place, so you only need 8. 100, 200, 300, 400 eg.

Total plates needed: 9*6 + 8*4 = 86 plates.

assuming they sing a hymn with all three numbers the same, the rest being near that number eg

331

332

333

334

then the highest count of the same digit is 9 on the same day. you have 10 digits, 0 – 9

10 * 9 = 90

therefore, keep 90 plates, 9 of each digit