FOSS gamedev and creative worlds

How many blocks are flying in the wire simultaneously for the whole server.

and

How many blocks are flying in the wire simultaneously per client.

Can someone explain to me what this phrase means?

Depends strongly on the context this is placed in what is meant.

I think it's referring to "the amount of node updates that are in transit between server and client". That number varies due to network latency, process intervals, update packet coalescing and various other forms. In general, it's likely not a lot (read: one), unless there's a mod that updates several nodes at once (e.g. mesecons or technic could easily cause 100 node updates to be sent at once)