3
The total number of blocks of 1s in the grid is **3**.

### Explanation:
1. **Two isolated 1s in row 12** at columns 2 and 6 form **2 separate blocks** (no connectivity).
2. **One large connected block** spans rows 13–20 in columns 2–5, with row 20 tapering to columns 2–4. These are connected vertically and horizontally, forming a single unified block.

**Total Blocks: 2 (isolated) + 1 (connected) = 3**.