I've found that 1ML has nearly twice as many nodes as other Lightning network researchers.
I think 1ML does not provide a data source, but can you guess the reason for the difference?
Next to 1ML, the number of nodes is almost the same.
My own c-Lightning nodes show about 4000 commands with the listnodes command, and lightning.chaintools.io uses the GetNetworkInfo-rpc call of lnd according to their source code.
Knot: 7,462 Channels: 39,310
Node: 4.011 Channels: 39,514
Node: 3,941 Channels: 38,983
Knot: 3,971 Channels: 39,201
Node: 3,978 Channels: 39,009