You'll probably have to rewire for 6V; leds have a Vf=3.4-3.5V, which means they need about 3.5V to give light (at least, a usable amount).
I suspect you're using 2 Lithium batteries (3V; CR123?), so you'd have to re-wire for 6V.
Most leds need about 10-15mA of current; but, when you want & get GOOD LEDs, the supplier should be able to provide a datasheet with technical information. However, settling for a current of 10-15mA, you can't go very far wrong.
Some drive them with 20 or even 30mA. Well, it's their cow... Some people drive their car at 200km/hr continuously. I don't.
Each LED needs its OWN resistor of about 6V-3.5V= 2.5V/.01A= 250ohm. Say, 220 ohm, which is a standard value. 220 ohm, 1/8W will be fine. You could put several LED/resistor units in parallel, to get the amount of light you'd need.
As far as to your question: my Dutch supplier won't be of much use to you.
My tip: do not buy batches on the web. I did. About 1/10 was either defect, or went defect very quickly. After a burn-in test, they seem to survive pretty well though. No more new failures.