Yes and no, I think they will build all the ponds given how far...

Yes and no, I think they will build all the ponds given how far they've come but I can certainly see an option to say not complete #6 & #7 if lower flow rates from less throughput allows slower pumping and thus more evaporation before it reaches pond 5. Only a short pipe-length to change later on in that case.

They'd still need to build ponds 8 and 9 though, so probably where they're at they'll simply build them all and be done with it. That way the expansion to higher throughput involves less parts and also no wait time for evaporation when the time comes.