The position vector of a point P is $\overrightarrow{r}=x\overrightarrow{i}+y\overrightarrow{j}+z\overrightarrow{k}$ , where

$x\in N,y\in N,z\in N$  and $\overrightarrow{a}=\overrightarrow{i}+\overrightarrow{j}+\overrightarrow{k}$ . If $\overrightarrow{r}\cdot \overrightarrow{a}=10$ , the number of possible position of P is