The new jets will be used in a few ways in the short and long term...

1. They will initially be used to grant much needed capacity to the Cityhopper network, in wake of the slot constraints at Amsterdam. This will be done by operating the E195-E2 on the most popular routes, and thus moving the E190 to take on the most popular E175 routes. Fleet optimisation will be worked out by 2021, to decide the precise network areas.

2. Some of the jets will probably replace the older 737-700 fleet... This will bring cost savings as Cityhopper crew have lower pay. As well as this, the 737-700 operates with 4 crew, whereas the E195-E2 will have 3 crew, thus bringing savings. Whilst the 737-700 fleet is young, returning the leased aircraft as the leases expire is probably cost effective, given that the E195-E2 will bring cost savings... In theory, KLM could phase out their entire 737-700 fleet with the E2 and the final remaining 737-800. I suggest this, due to an initial suggestion that KLM will remove the 737-700 by 2022, although this is perhaps a little delayed now.

3. In the really long term, KLM will have an eye on using the E195-E2 to actually expand at Amsterdam. Within the next 7 years, I can expect to see a rising slot limit at Schiphol, and KLM will be looking at the later orders and options to utilise these new slots for expansion. KLM could (in theory) use their initial 16 E2s to phase out the 737-700, and then have 5 remaining E2s to expand once AMS can expand... This isn't including options. This is also a really long term view, and a view that isn't guaranteed to take place due to AMS uncertainty.

I have also heard some (unconfirmed and perhaps unlikely) rumours that KLM could potentially lease 2-3 planes to Air Antwerp in the coming years to enable expansion and replacement of the elder Fokker 50. This would perhaps come from an E175 which would be displaced by an E190, thus displaced by an E2.

(Note - everything mentioned here is simply speculation / rumours which I have heard on my travels)
-G