Was playing with the updated schedule and pulled in weekday departures for Delta and Delta Connection flights for nonhub cities to see how they stack up. I didn't make any attempt to quantify by capacity or type, and some cities with relatively high frequency (for example RDU and IND) are about 70% Delta Connection while others (for example LAS) are nearly all mainline. This is just weekday departures for flights marketed primarily as Delta or Delta Connection.

I didn't count the hubs of ATL, DTW, MSP, SLC, MEM, CVG, LAX, LGA or JFK. This is about knowing how many flights airports below this tier have.

1. 72 Boston
2. 58 Chicago ORD
3. 55 Raleight/Durham
4. 53 Washington DCA
5. 51 Orlando
6. 46 San Francisco
7. 44 Dallas/Fort Worth
7. 44 Seattle
9. 42 Indianapolis
10. 40 Las Vegas
11. 36 Columbus
11. 36 Charlotte
13. 35 Baltimore
14. 35 Philadelphia
15. 34 St Louis
15. 34 Pittsburgh
15. 34 Nashville
18. 32 Kansas City
19. 30 Milwaukee
19. 30 Tampa
19. 30 Houston IAH
22. 29 Denver
22. 29 Fort Lauderdale
22. 29 Newark
25. 27 Phoenix
25. 27 New Orleans
27. 26 San Diego
27. 26 Buffalo
27. 26 Richmond
30. 25 Louisville
31. 24 Washington IAD
32. 23 Norfolk
32. 23 Miami
34. 21 Grand Rapids
34. 21 Cleveland
34. 21 Portland
34. 21 Jacksonville
34. 21 Omaha
39. 20 Hartford/Springfield
39. 20 Madison

I believe that's everyone at 20 or higher. I didn't quite hit 20 at several others (19 at SAT, AUS, TYS, 18 at LEX, etc) but I think I went deep enough to hit everyone at 20 or higher.