I guess SC didn't really think that anyone would store 20+ boosters when they introduced them. However, they've been adding more and more chances to get "free purchased" boosters ever since. I have some 80 boosters and I've been afraid of the same click error (?) for a long time.
My advice is to keep booster count low. Under 10. I really don't know why I'm hoarding them myself.
Edit: 10th booster would cost 45 diamonds to keep so maybe one should keep the count even lower.
Edit2: Here's the whole price list:
Booster# Diamonds
1. Free
2. Free
3. Free
4. 2
5. 4
6. 6
7. 12
8. 20
9. 30
10. 45
11. 60
12. 80
13. 100
14. 150
15. 200
16. 250
17. 300
18. 350
19. 400
20. 450
21.+ 500