"How long can you last with 50% mana before you get critically low? Oh also, how taxing is it for you to transform into a different age... and how long can you last in your figurine state? And is transforming into a figurine more or less difficult than appearing a different age? and how difficult... I'll shut up and let you start to answer." that nap apparently let your subconscious deal with stuff enough to focus on logistics rather than what if scenarios. You even mustered the capacity to grab a notebook and a pencil, taking notes as you asked follow up questions.
Normal activity: 5% mana usage per day can last 20 days, heavy usage will deplete faster (constant work, heavy lifting, complex tasks)
Figurine transformation: 5% of mana capacity to transform, 0.01 of mana capacity per day meaning they can stay in that state for up to 24 years before needing to transform back to feed again.
Age transformation: 3% of mana capacity, becomes new form, requires no additional mana beyond that of activity assigned
Soul Exchange: Cast by master, uses Master's mana.
Subsumption: Adds 5% to daily mana usage.
"And how quickly am I generating mana? You said I could sustain everyone so I can at least create 50% of your average capacity per day, right? I can hold roughly the same amount of mana as one of you, but its not good enough to hold the mana, I need to generate or collect it or whatever for you guys. Enough to cover everyone's standard daily usage?" You asked trying to cover all your bases before making plans.
"You have regained approximately 10% of your mana reserves" one of them said which you thought was sufficient though you couldn't be sure because you forgot to look at the clock and you don't know if you slept for 1 hour or 4...
"For everyone's safety I'd like to put some of you back to figurine form." you announced expecting to be yelled at or at least hear some disappointment but none came. "Any volunteers?" 10 hands raised... You immediately took it personally that nobody wanted to remain your servant. "Doesn't anyone want to stay with me?" 10 hands raised as you stared dumbfounded. "Duh, " you said remembering these are animated creatures, not sentient creatures. You will have to pick who stays and who goes... if you can maintain the quantity you have active, you can always go back and animate another.
Actually, another better question would be how many naked teen boys does he want to try to hide from his parents? Without the homunculi actively opposing him returning them to their figurine form, he might be best to keep just one active for testing spells, trying to grow his mana regeneration and capacity, make sure depleting his mana doesn't have any strange side effects, and work his way up from there.