How do damage rolls work in Little Cups in regards to Bullet Seed?
So all multi-hit moves are done by calculating base power of the move and then doing the roll for each hit, which is why every hit can do different damage or crit independently. Because most multi-hit moves have low base power, the way the formula works usually ends up with a series of 15 "low damage" rolls and 1 "high" roll. The reason for this is that the random factor (a multiplier of .85 to 1.0 which is what causes the 16 possible damage rolls) is applied before other multipliers when calculating. Since everything rounds down basically immediately, any max damage less than 7 will result in only two possible damage results - the high roll and 15 ones that are one damage less. This is because while 6*.99=5.94 and 6*.85=5.1, both of which round down to 5, 7*.99=6.93 and 7*.85=5.95, which rounds to 6 and 5 respectively. This effect is the origin of LC's low and high rolls, and the variance between rolls only increases as base damage goes up, until at 100 damage all 16 rolls have a different value.
Beyond this, since most multi-hit moves in LC, Bullet Seed included, have such low base power they rarely are able to achieve that 7 damage per hit on a neutral target they generally have only two rolls, the 15 low ones and the 1 high. This can change if you boost your attack high/lower your opponent's defense enough, but in most cases that's what happens, you'll have two damage rolls and one of them will usually be 1 more damage than the other.
This changes, of course, when SE or NVE multipliers are taken into effect. For example, here's Pumpkaboo-Super's Bullet Seed on standard Mienfoo.
0 Atk Pumpkaboo-Super Bullet Seed (3 hits) vs. 0 HP / 196 Def Eviolite Mienfoo: 9-12 (42.8 - 57.1%) -- approx. 12.1% chance to 2HKO
(3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4)
Here's Bullet Seed on that same Mienfoo, only modified to be Water-type.
0 Atk Pumpkaboo-Super Bullet Seed (3 hits) vs. 0 HP / 196 Def Eviolite WaterFoo: 18-24 (85.7 - 114.2%) -- approx. 6.3% chance to OHKO
(6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 8)
You can see how the rolls are actually just multiplied by 2, so now the high roll is 2 more than the normal roll. Here's what happens on a NVE hit, aka GrassFoo.
0 Atk Pumpkaboo-Super Bullet Seed (3 hits) vs. 0 HP / 196 Def Eviolite GrassFoo: 3-6 (14.2 - 28.5%) -- approx. 0.1% chance to 4HKO
(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2)
Again, this makes sense - the rolls on the normal Mienfoo are divided by 2 and rounded down - the '1.5's from the 3s become 1s. Now let's see what happens with Grass/Steel Mienfoo for a quad resisted attack.
0 Atk Pumpkaboo-Super Bullet Seed (3 hits) vs. 0 HP / 196 Def Eviolite FerroFoo: 3-3 (14.2 - 14.2%) -- guaranteed 7HKO
(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1)
Since the game really hates giving out 0 damage rolls without Reflect (afaik it's flat out impossible without a screen) everything ends up rounding down to 1, resulting in no effective damage variance.
In general, multi-hit moves are stronger than non-multi hit moves. Unless the user is particularly weak, the target is very bulky, or you're using the attack on a resisting target you'll often get 2-3 damage rolls, meaning a minimum of 4 damage and a maximum of 15, not even taking into account the additional critical hit chances, and of course the move can be 3-4 or 4-5 instead.
Hopefully that answers your question enough, let me know if there was something in particular that I missed that you were confused about!