An instantaneous heater just can't produce the hefty flow which some
people like (basic physics intervenes: water just takes a lot of energy
to heat up!). So, with a 9kW electric-instant, you're not going to get
more than a 'few' (can't be bothered doing the calculations yet again!)
litres/min; with a combi which goes up to say 24kW you'll get more -

So here's the amazingly detailed back-of-an-envelope calc. Takes
4.2kJoules to raise 1 litre through 1 degree C. A reasonable midpoint
for how much temp rise you want is 30 degrees - from 10 to 40; so
handwave it to 120kJ for each litre. A nice round number, that, since we
want our final answer in litres/minute - since 1kW means 1kJ/sec, to get
120kJ in a minute we want 2kW.

Thus, rule-of-thumb-close-enuff-for-jazz says 'shower delivery rate, in
litres/min, = half of kW rating'.

Sanity-check: that says a 9kW leccy shower puts out 5L/min. That's
rather below the mfrs advertised figures - e.g. Goolgle says
dealtime.co.uk describe several 10.5kW showers asdelivering "up to"
14L/min. Either there's a mistake in basic physics... or, the "up to"
figure is for a lukewarm shower (30mumble) in summer (incoming at
20mumble, at least until the prewarmed water in the house cold water
pipes has all come through and you drop back to the underground temp
closer to the low teens). Second sanity-check: 1min for a litre through
30degs with 2kW input says a 2kW kettle will take 3 mins to boil (90
degs rise needed) - yup, that's consistent with normal experience. Third
sanity-check: combi specs at the top of my Google results for 'combi
"litres per minute"' say '28kW 11.4 lpm' and '27kW 11 lpm', for a
35degree rise; perfeckly in line with the rule-o-thumb derivulated above.

So that's two-out-of-three sanity checks in line with Normal Fizziks,
and one out in creative-marketing-specmanship land. Pretty normal, then ;-)