they are completely different chips, to start with isn't the atmega used 8 bit? (vs Axoloti 32bit),
so comparing clock speeds is meaningless.
(also shruthi is using a different filter board, so that wont load the cpu)
as you say
you could create lots of voices if the voice was simple (e.g. saw with simple filter), but make a complex enough oscillator with complex filter, and fx and you might be down to one!
so impossible to generalise...
but to give you an idea...
if I'm careful, and not crazy stupid with fx, I can get 16 voices of reasonable oscillator/filters.
if I want more complex fx, and perhaps complex filters this sometimes will drop down to 8.
e.g. some of the mutable instruments oscillators/fx will knock your voice count down...
often now, I put the main voices on one Axoloti, and then feed its output into another Axoloti for the FX.
have a look at my you tube channel and others Axoloti users , and you will get an idea of what is possible.
(or should I say, what we want to use our Axoloti for, rather than is possible )
its wise to keep expectations 'reasonable', especially when we are all used to very powerful pc, an Axoloti is a microcontroller, which is many times less powerful that your average desktop computer.
with that said, I was surprised how much was possible with Axoloti when I first got mine, particularly for the price... and its standalone nature is awesome.
anyway, as only you know what you have in mind (and your patching/code skills can play a part too), really you can only know what is possible by trying.
I also found once I got patching with Axoloti, my idea of what I was going to use it for changed/evolved, I to play to its strengths.
EDIT: btw, when I refer to voices here, its worth pointing out, often these will have multiple oscillators in them - so even my simple 16 voice patch. will have a few oscillators inside, for detuning, or sub oscillators, etc. ... and of course the number and type of oscillators within one voice, will have a big impact on cpu... (e.g. think about FM, and how many oscillators you might use in one voice for that!)