I regularly use a TI-89 Titanium and HP 50g both (each does some things better than the other), occasionally along with my old HW2 TI-89 for multitasking two programs. That makes three.

Nine for me, a la youtu.be/4Mjn98Bs2Cg

Which ones?
I've used my TI-84+SE and my TI-89Ti simultaneously, by the way.

Haha, nice. A couple more steps forward and we could have a whole TI calculator server farm. :-D

It was 2 for me as far as I can remember and probably on several occasions. (Ti-68k calcs)
I guess I was playing a link cable game on both :P

Yes, I also answered "Two", testing link play. Those people who answered more than two probably don't have the same definition of "simultaneously" we have. ;-)

If emulators count, an emulated TI-80 (to look for glitches) & 2 real TI-80s (to test on both known ROM versions). If not, very probably 2 TI-80s & a TI-89 Titanium at some point.

Not too long ago, I was experiencing some weirdness with the 68K calcs' random number function, and, being in one of my occasional wacky and crazy moods, ran four instances of TIEmu on my quad core i5, running TI-BASIC programs to brute-force the RandSeed arguments until I found the range of values that acted up. Only took a couple of weeks to try arguments 0 to 1,000,000,000. That would have taken around 18,000 hours of actual operating time on a single real 89.

If you do not mind telling, what was the weirdness & the relevant range of parameters?

Above 1,000,000,000, certain ranges of consecutive integers to RandSeed actually end up seeding the internal RNG with the same value. And if you try seeds like 1, 2, 3, etc., and compare the sequences, they don't look terribly random from one to the next. It seems the seeding algorithm was far worse than I expected, much to my annoyance. I was trying to make a random sentence generator-type program where each saying was identified by the seed value, so you could go back and retrieve a particular saying in the future if desired. But it doesn't work too well when going through consecutive seeds keeps revealing similar patterns of sentences over and over.

I'm actually surprised that 2 users reported using infinity calculators simultaneously.

That choice got me to thinking...with the right setup, you could probably get infinitely many emulators running, although they would either have to be all identical (which is kind of cheating) or follow a simple pattern in order to be able to do it with a finite amount of real computation.

I didn't think that was possible. How could you do that?

As a sidenote, I also attempted to send Sunrise 2 from my TI-84 Plus Silver Edition to a friend's TI-84 Plus.

Perhaps running a calculator in a calculator. :P

announcing the TI nCeption

Emu8x - see the URL attached.

That looks pretty interesting. Would that count as running several calcs at once? :P

I wouldn't think so. From a software point of view, yes; it is emulating a calculator, but the functionality of the host is completely absent. And from a hardware point of view, there's only one physical calculator.

I'm surprised no one answered zero or one. I guess we're all geeks. ;-)

Of course no one here would answer zero. :)

Oh, yeah. :-) I think I really meant to say one, not zero and one. My brain apparently executed a "<=" opcode instead of "==" for some reason.

Haha! My mind just pictured a guy going here thinking, "TI Calc. I don't know what a calculator is, but this site seems relevant to my interests." Then he would stereotypically say "Doi, doi, what's √cos(sin(30))^-1)/5?, doi doi"