The number of integral values of $p$ for which the vectors $(p + 1)\hat{i} - 3\hat{j} + p\hat{k},\; p\hat{i} + (p + 1)\hat{j} - 3\hat{k}$ and $-3\hat{i} + p\hat{j} + (p + 1)\hat{k}$ are linearly dependent, is ______.