They would be fairly visible, but note that the horizontal angle of half intensity is 100° while the vertical angle is only 40°, so mount them accordingly.
When you have more than one LED, they will be either in series, in parallel or a mixture (unless you use a separate power source for each.
To reduce power consumption, you need to have 2 LEDS in series on 9V (3 LEDs are too many, considering the voltage variation of a 9V battery throughout its lifetime).
However, a 9V PP3 battery, if that's what you're hoping to use, is a very poor solution, as it will only run a single LED (@20mA) for around 20 hours, so 20 LEDs will only have power for about 1 hour, give or take - 2 hours if you put each 2 in series (it will probably only be around half that, since capacity drops with a rising load).
AA cells have more than 5 times the capacity, so would be a better choice.
A wall wart supply would be the optimum solution of course.
A 1.5V battery cell is actually closer to 1.65V when fresh and considered flat when at 0,9V. A 9V battery varies from 9.9V down to 5.4V over its useable life, which means that the light output from your LEDs will fade if a resistor is used to control the current.
Eg. with the closest E12 value from your 345 Ohm resistor, which is 330 Ohm:
from (9.9V-2.1V)/330 Ohm = 30 mA
to (5.4V-2.1V)/330 Ohm = 16.4 mA
Constant current devices are the key to a uniform light intensity over the battery lifetime.
Should all 20 LEDs be on at any given time or what?