It's more like this:

Practices Gained Per Level:
16 = 1 practices per level
17 = 2 practices per level
18 = 3 practices per level
19 = 3 practices per level
20 = 3 practices per level
21 = 3 practices per level
22 = 4 practices per level
23 = 4 practices per level
24 = 5 practices per level
25 = 6 practices per level
Note: Wisdom points achieved by items count as 1 wisdom point contributing to practices per level.