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According to the ros::AsyncSpinner Class Reference (from the roscpp API docs):

ros::AsyncSpinner::AsyncSpinner(uint32_t thread_count)

Simple constructor. Uses the global callback queue.

Parameters: thread_count: The number of threads to use. A value of 0 means to use the number of processor cores

According to the ros::AsyncSpinner Class Reference (from the roscpp API docs):

ros::AsyncSpinner::AsyncSpinner(uint32_t thread_count)

Simple constructor. Uses the global callback queue.

Parameters: Parameters:

      thread_count: The number of threads to use. A value of 0 means to use the number of processor cores

According to the ros::AsyncSpinner Class Reference (from the roscpp API docs):

ros::AsyncSpinner::AsyncSpinner(uint32_t thread_count)

Simple constructor. Uses the global callback queue.

Parameters:

      thread_count: The number of threads to use. A value of 0 means to use the number of processor cores

void AsyncSpinnerImpl::start()
{
  [..]

  for (uint32_t i = 0; i < thread_count_; ++i)
  {
    threads_.create_thread(boost::bind(&AsyncSpinnerImpl::threadFunc, this));
  }
}

According to the ros::AsyncSpinner Class Reference (from the roscpp API docs):

ros::AsyncSpinner::AsyncSpinner(uint32_t thread_count)

Simple constructor. Uses the global callback queue.

Parameters:

      thread_count: The number of threads to use. A value of 0 means to use the number of processor cores

Edit:

Then the question becomes, what would happens if I use 1? Does that mean the callback is still in the same thread as the main program? will the callback blocks the main loop?1?

From the source of AsyncSpinnerImpl::start() (lines 143-167):

void AsyncSpinnerImpl::start()
{
  [..]

  for (uint32_t i = 0; i < thread_count_; ++i)
  {
    threads_.create_thread(boost::bind(&AsyncSpinnerImpl::threadFunc, this));
  }
}

In this, thread_count_ is set in the ctor of AsyncSpinnerImpl to be either equal to the nr of cpus, or to the value passed in via the ctor of AsyncSpinner.

According to the ros::AsyncSpinner Class Reference (from the roscpp API docs):

ros::AsyncSpinner::AsyncSpinner(uint32_t thread_count)

Simple constructor. Uses the global callback queue.

Parameters:

      thread_count: The number of threads to use. A value of 0 means to use the number of processor cores

Edit:

Then the question becomes, what would happens if I use 1?

From the source of AsyncSpinnerImpl::start() (lines 143-167):

void AsyncSpinnerImpl::start()
{
  [..]

  for (uint32_t i = 0; i < thread_count_; ++i)
  {
    threads_.create_thread(boost::bind(&AsyncSpinnerImpl::threadFunc, this));
  }
}

In this, thread_count_ is set in the ctor of AsyncSpinnerImpl to be either equal to the nr of cpus, or to the value passed in via the ctor of AsyncSpinner.

Does that mean the callback is still in the same thread as the main program? will the callback blocks the main loop?

If you specify a thread_count of 1, a single additional thread is created. So no, iiuc, the callback processing should not happen in the 'main loop'.

PS: I would've found it rather odd for a class called AsyncSpinner to return to synchronous behaviour only be changing the nr of threads it uses.