No, according to RAW, but I would argue for a special provision
According to RAW, concentration on a spell is all-or-nothing. There is no in-between where you can maintain only part of a spell, such as a certain section of a Darkness spell or a certain area of an Entangle spell.
However, given that the spells specifically mentions that it summons up to eight demons, you might be able to argue that the full spell still holds if less than eight demons are present, which is the case as one demon can be reduced to 0 hp and disappear without the others also vanishing. In that case, your DM could allow you to make a certain check (I would go for Int or your casting stat), to be able to isolate the part of the spell that governs the number of demons, and release one demon from that binding while maintaining control of the rest. Note that this would not allow you to add a demon should you have less than 8, as you would have to restart the entire summoning spell and thus lose the demons you would already have.
This provision can be rephrased in the following way:
The flavor of the spells sends the message that, although the spells summons a certain number of demons, each demon is individually bound to the spell, which is why it doesn't fail automatically when one of them is killed. Therefore, a caster with sufficient mastery over the spell (established here by a check using either your understanding of the spell's inner working (Int) or your magical proficiency (casting stat)), could release a certain number of demons from the summoning contract without canceling the entire spell. It's akin to firing one or two employees instead of shutting down the entire company.