Reading the table from the Summon Lesser Demon spell (XGE pg. 167):
You utter foul words, summoning demons from the chaos of the Abyss. Roll on the following table to determine what appears.
d6 Demons Summoned 1-2 Two demons of challenge rating 1 or lower 3-4 Four demons of challenge rating 1/2 or lower 5-6 Eight demons of challenge rating 1/4 or lower
Is it possible to summon only two manes (CR 1/8) if I roll 1-2?
When casting Summon Lesser Demon, you have to roll a d6: if you obtain a 1 or 2, the table says that the spell summons exactly 2 demons of CR 1 or lower. Since manes' CR is 1/8, and 1/8 is less than 1, the DM may make the spell summon 2 manes, per description:
The DM chooses the demons, such as manes or dretches, [...]
One may find more information in the Sage Advice Compendium about DM choosing the creatures summoned by a spell once the CR has been established:
When you cast a spell like conjure woodland beings, does the spellcaster or the DM choose the creatures that are conjured? [...] Some spells of this sort specify that the spellcaster chooses the creature conjured. For example, find familiar gives the caster a list of animals to choose from.
[...]
The design intent for options like these is that the spellcaster chooses one of them, and then the DM decides what creatures appear that fit the chosen option. For example, if you pick the second option, the DM chooses the two elementals that have a challenge rating of 1 or lower. A spellcaster can certainly express a preference for what creatures shows up, but it’s up to the DM to determine if they do. The DM will often choose creatures that are appropriate for the campaign and that will be fun to introduce in a scene.
You could get two Quasits (CR: 1), two Dretches (CR: 1/4), two Manes (CR: 1/8), or some combination thereof.
Or if you have a comfortable DM maybe you get a Barlgura toddler or a Chasme without wings.
It's helpful to understand who gets to choose what. The DM gets to choose the exact demon... this is a choice given to the DM to allow it to make sense for the current campaign setting and situation, and also enable the DM to create names and personalities and so forth for the demons that get summoned. But the caster is the one who chooses the CR, within limits specified. Depending on the dice roll, they have a ceiling on the CR they are asking for; on a 1-2, they can get at most a CR 1 creature, but if they WANT a weaker demon, they can lower the CR they get voluntarily.
So if the caster rolls a 1 on the d6, but wants two manes, they can ask for CR 1/8 instead of the maximum CR 1 the spell allows. But it's the caster that makes this choice, not the DM.
The alternative reading (where the DM gets to simply summon up a bunch of manes no matter what the caster wants) makes very little sense from a game design perspective.
