For the Bitnodes project (https://getaddr.bitnodes.io), I am using only 1 instance of the crawler which connect/disconnect from reachable nodes periodically. This should be sufficient to aggregate network metrics. Running more than 1 instance and taking up connection slots from peers will reduce the number of available open slots for actual peers to connect to. There are only 125 open slots available per node unless of course if you have configured it manually with maxconnections option in bitcoind.conf.