Assuming a +0 Cha modifier, the Leadership score is Lvl x2 -5. This works from 6th through 15th, and doesn't matter afterwards because the Leadership table caps at 25. Assuming that all thralls and believers capable of entering the Thrallherd PrC do so, and that no early entry tricks are used, this results in a total of 4225 thralls and believers directly or indirectly loyal to the thrallherd at 20th level.
Level 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Total Net total
1 0 0 0
2 0 0 0
3 0 0 0 0
4 0 0 0 0 0
5 0 0 0 0 0 0
6 0 0 0 0 1 1 1
7 0 0 0 0 0 1 1 2
8 6 0 0 0 0 0 1 7 9
9 10 1 0 0 0 0 0 1 12 21
10 20 2 1 0 0 0 0 0 1 24 45
11 30 3 1 1 0 0 0 0 0 1 36 81
12 40 4 2 1 1 0 0 0 0 0 1 49 130
13 60 6 3 2 1 1 0 0 0 0 0 1 74 205
14 90 9 5 3 2 1 0 0 0 0 0 0 1 111 317
15 135 13 7 4 2 2 0 0 0 0 0 0 1 1 165 689
16 135 13 7 4 2 2 0 0 0 0 0 0 0 1 1 165 1173
17 135 13 7 4 2 2 0 0 0 0 0 0 0 0 1 1 165 2029
18 135 13 7 4 2 2 0 0 0 0 0 0 0 0 0 1 1 165 3369
19 135 13 7 4 2 2 0 0 0 0 0 0 0 0 0 0 2 0 165 4225
20 135 13 7 4 2 2 0 0 0 0 0 0 0 0 0 0 2 0 0 165 4225