CH> Do as I did and hook up more LEDs than you think it will light 
 CH> in series, then remove one at a time until they light. Then 
 CH> remove another and see if they brighten, and repeat as needed 
 CH> till they don't brighten anymore. Then adjust the number to the 
 CH> desired brightness, as long as it's at least one more than 
 CH> maximum brightness, so they aren't totally over loaded. (I went 
 CH> 2 steps down in brightness, and they outlasted my use for them.
This is a silly way to figure it out...
If you want longevity the thing to do is to figure out what current you want 
to feed 'em.  I've found that 20-25 mA is a pretty good number,  and tends to 
give long life.
Then you assume (or measure!) a typical voltage drop,  I've found that 1.8v 
is fairly typical,  though the green ones may be a little higher and the IR 
ones a little lower.
The rest is simple ohms law...
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