so what is the problem?
note that LEDs are nonlinear so you need to use series resistor. say you have LEDs that draw current If and have forward voltage drop Vf.
it is typical for LEDs to be somehting like
If=20mA
Vf=3.2V
note that we need some "room" when it comes to voltage drop across resistor for it to do it's job. ideally we would want to minimize this for better efficiency but that won't always work.
something like 1V should be bare minimum because voltage of vehicle battery fluctuates. the more voltage is across resistor the more linear response you get and less change in LED brightness when battery voltage fluctuates, but as mentioned this reduces efficiency.
for example if your bike uses 12V battery then you can have up to 3 LEDs in series in this example because that would get you to
3*Vf=9.6V and that leaves some 2.4V for resistor.
if you add one more LED, you are out of luck because this would have 4*Vf=12.8V which will not allow LEDs to lit up brightly even without resistor.
if you have more LEDs, you can use them in groups of 3 (connected all three in series and with a series resistor) but you can power each group in parallel.
now to make a strobe, you will need something that will produce several pulses (strobe).
this can be done using different methods. you could use microcontroller (they require programming) or a pure hardware solution with 555 as oscillator and 4017 as counter and decoder, something lsimilar to circuit like this:
**broken link removed**
but some people have already done it too, here one example:
**broken link removed**