There's a cap on how many entries a single user can hold shares in per contest, based on the contest size:
Contest size | Max entries you can hold shares in |
1–12 entries | 1 |
13–36 entries | 2 |
37–100 entries | 3 |
101+ entries | 3% of total entries, or 150 — whichever is smaller |
If a purchase would push you over this limit, it will be blocked.