Liquiducts have two modes. Passive and active/pump mode (active mode you wrench it to get the arrow and apply redstone power to it).
In passive mode they move about 80 steam a tick. For reference gold pipes move 40 steam per tick.
In passive mode you can combine it seems an unlimited number into one pipe. I have run all the steam off 36 HP boiler (720) by connecting 9 passive pipes to a side of the boiler and combine into one liquiduct down to the engines.
The active mode it seems to double the throughput,
but doesn't combine into one pipe well. Example two independent active runs delivery 320 steam to a turbine (160 each duct), but i can't get them to combine into one input at the turbine (it may not accept that much through one block).
Edit -- tested it and it combines fine, it is on the turbine end. It will only accept 160 steam input in any given block, so you just have to split into two at the turbine.
320 steam to the turbine: