Regarding whether you "need" more than 49 keys, 49 keys are sufficient for playing all the notes on the grand staff plus a few ledger lines above and below without needing to use the octave-shift buttons, so it is-- in my opinion-- the minimum number of keys you should consider for playing with both hands at once.
Longer keyboards-- with 61, 76, or 88 keys-- are even better for playing with both hands at once, and the longer the better. A keyboardist who's happy playing with 61 keys and who then buys a keyboard with 76 or 88 keys generally doesn't want to go back to 61 keys if it can be helped.
Shorter keyboards-- with 25, 37, or 44 keys-- can be sufficient for playing leads or bass lines with one hand, as well as to use for creating or triggering beats and loops with a DAW and other software, especially if desk or lap space is an issue and greater portability is desired.
So as far as the number of keys, that really depends on whether you have other keyboards or this will be your only one, how many hands you expect to play with, what you plan to use it for, how portable and compact you need it to be (such as whether it needs to fit inside a backpack), whether you're going to set it up in a semi-permanent spot in a studio or will be setting it in your lap and playing it while you sit in a comfy chair, and other considerations.
For example, if you're looking for a keyboard that you plan to use while taking piano lessons, you'll be much better off with 76 or 88 keys. Or if you want a keyboard that has weighted piano-like keys, you'll need to look for 76 or 88 keys because you generally won't find weighted keys on a keyboard having fewer than 76 keys.
But for your purposes, I think 49 keys might be adequate for a starter keyboard, especially if it isn't too expensive.
Regarding which of those three is best, I have no personal experience with any of those brands, so I won't offer an opinion about which is best. If you can't try them out in person to see how well you like the way their keys feel, then I would recommend reading online reviews from people who actually bought them.
Note that all three keyboards come with a DAW-- PreSonus Studio One Artist (with the Acorn), Ableton Live Lite 9 (with the Alesis), and Bitwig Studio 8-Track (with the Nektar). If the bundled software will be one of the deciding factors then I recommend researching the specific capabilities and limitations-- especially the limitations-- of each of those DAWs and any additional bundled virtual instruments software.
You can get PreSonus Studio One Prime for free-- although Artist is better than Prime as far as capabilities and limitations-- so you might want to install and register Studio One Prime to see what you think of it before ruling it out, and study the chart on PreSonus' web site that shows the differences between Prime and Artist. One thing you should be aware of is that neither Prime nor Artist let you use third-party plug-ins, although Artist does let you purchase a module that lets you use third-party plug-ins.
Ableton Live Lite is the least-capable and most-limited version of Live-- even less capable and more limited than Live Intro, which is the least-expensive paid version of Live. In contrast, Studio One Artist is the least-expensive paid version of Studio One-- so in terms of how much you'd have to pay for the DAW if it weren't bundled with the controller, I'd say that the Acorn gives you a better DAW deal than the Alesis does. On the other hand, Live lets you use third-party plug-ins, which is good because Live Lite doesn't come with a lot of included instruments as compared to what you get with Live Standard or Live Suite. And Studio One doesn't have a clip-launching mode such as the one that Live is famous for.
I've heard of Bitwig Studio and have a demo version on my computer, but I hadn't heard of 8-Track until today. It appears to be available for free from Bitwig Studio's web site, so I'm sure I'll be installing it sooner or later. Bitwig does have a clip-launching mode similar to Live's, so you don't need to get Live just for that feature. And it looks like Bitwig also supports MPE (MIDI Polyphonic Expression), which might not be available on any of those three controllers but I think you should still be able to make use of it in the DAW. But if 8-Track is free (which I haven't verified yet) then you could of course get the Alesis and download 8-Track to use with it.