How to configure parameter reexec of debug_traceTransaction?

Question

I am trying to use debug_traceTransaction to scan the internal transactions of smart contracts, so I need set the value of reexec, what value should I use? It seems that reexec is the number of blocks the EVM need to re-execute to generate the desired state.

    web3.currentProvider.sendAsync({
      method: "debug_traceTransaction",
      params: [txHash, {tracer: "callTracer", reexec: **??** }],
      jsonrpc: "2.0",
      id: "2"
    }