How do I know how many number of processor run on tbb code?

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Linux

public:
void operator()(blocked_range<int> r) const {
for (int i = r.begin(); i != r.end(); ++i) {
for (int j = 0; j < size; ++j) {
for (int k = 0; k < size; ++k) {
c[i][j] += a[i][k] * b[k][j];
}
}
}
}
};


int main()
{
// Initialize buffers.
for (int i = 0; i < size; ++i) {
for (int j = 0; j < size; ++j) {
a[i][j] = (float)i + j;
b[i][j] = (float)i - j;
c[i][j] = 0.0f;
}
}

// Compute matrix multiplication.
// C <- C + A x B
parallel_for(blocked_range<int>(0,size), Multiply());

What I have tried:

can we explicit enter the number of CPU while compiling?