Assuming I'm understanding the question right:
The most IVs a Pokémon can have in a stat is 31. Therefore, if you want a Pikachu with the highest possible HP stat at levels 10, 11, or 12, you'll want to catch one with 31 HP IVs. Here are the HP stats of 31 IV Pikachu at the three specified levels:
Level 10 — 30
Level 11 — 32
Level 12 — 34