By default, when I don't set any limit for the arg_in_peers via RPC command, the value will be -1. Does this mean my node cannot accept any incoming connection, or there will be no limit on this number? In other words, what is the default value of max_in_connection_count?
The default value for in-peers is -1 (unlimited). The reason -1 is (in effect) unlimited is because
max_in_connection_count is an unsigned integer and therefore 0-1 wraps to the maximum of an uint32_t (i.e.
If you need to limit the inbound connections, set
in-peers=N (replacing N with the number of incoming peers to allow) in your daemon config.