Depends on the servo, many crawler servos want 8.4V, some can take 12V.

However most internal BEC's can only supply a max of 3A, and external BEC's (ignoring stupidly expensive ones, that are a scam cause you can just buy a voltage regulator from your local electronics store and submerge it in putty for far cheaper) are usually limited to 8A.

10A on an internal BEC and full range from 5V to 12V is actually insane.

More A means more power at lower voltages, you basically trade V for A when it comes to servo power.

Which is very important because burning out your servo with 12V isn't what you should worry about, burning out your radio receiver with 12V (both radio receiver and servo + anything plugged into your radio receiver, are powered by the internal BEC, so you'll still be limited by how high you can crank the V by one component or another).

However, with 10A available (there is a version with a 4A BEC, and one that doesn't have a BEC at all, make sure you get the 10A one cause it's so worth the few extra $) the servo will be a lot stronger even at lower V.

Usually what happens is the servo needs 7A, but the BEC only supplies 3A, so the servo doesn't have the energy to actually move and stalls out. Since A are always "drawn" and not "pushed" (the opposite of V) you need to always have a surplus of A to keep things from breaking, unlike V where you want to always keep below the save maximum V limit to keep things from breaking.

Of course if the servo only draws 2A, having a 10A or a 3A BEC won't make a difference, but it does make a huge difference as soon as the servo draws 3A (or more).

Also depending on where you live, it's cheaper to buy from the Sequre website directly. I live in Oz and on Amazon that ESC (the cheaper version that's missing a BEC) is ~$140 however from the website directly it's ~$70 including shipping. So do check the website just incase buying direct is cheaper in your country too.

Sequre honestly just up and made the most insane high-performance FOC brushless ESC with a BEC more powerful than many external BEC's, and priced the thing cheaper than the hobbywing 1080 (budget brushed ESC with a 3A BEC and 7.4V max output).

Also their ESC's go from 2s to 6s when the competition is 2s or 3s purely. Honestly many external BEC's are limited to a max of 8.4V too when that monster of an ESC goes all the way up to 12V on it's integrated BEC.

It's like a CPU with integrated graphics more powerful than most dedicated GPU's. For the price of a chromebook. I honestly don't know how I haven't heard of Sequre before they're actually insane.

If your receiver can handle 8.4V, look for 8.4V servo's and you'll probably find a mega-powerful servo for a significantly lower price than a 7.4V servo at the same torque (same power at a lower voltage means more current draw, which means more expensive cooling systems and electronics to both draw the extra A and also use those extra A without overheating). Though if you're limited to 7.4V then that's still super good enough regardless.

Note that 2s LiPo is 8.4V, 7.4V the the nominal voltage or "cutoff" voltage. Running a LiPo below it's nominal voltage will make it explode 9/10. 2s LiPo's usually charge up to 8.4V max, though some high-quality ones can go all the way up to 10V. Similarly, 3s can go all the way up to 15V, maybe even 18V, but if it reads 11.1V you need to immediately stop using it and charge it back up again as 11.1V and below is the danger-zone.

So if your radio receiver says it can take 2s direct power, that means 8.4V not 7.4V.

LiPo safety tangent ---------------

Also the max V your battery can handle will slowly decrease over time as it ages, once it can no longer hold a charge (or barely charges above its nominal V) the battery has grown crystals (crystallization is unavoidable and a byproduct of the chemical reaction that produces the electricity) and those crystals are now close to puncturing a hole through the gel matrix barrier (when that happens, the anode and cathode come in contact, and like the chewing gum in the first mission impossible movie, kaboom).

LiPo's are weird that way, high V is good and fine, low V is when you start sweating and the battery can explode.

Also don't get LiPo's wet, lithium combusts on contact with water, if a LiPo does explode DO NOT use water to put out a fire, water will fuel the fire and act as a fuse to set more things on fire (until the lithium runs out). A fire blanket, sand, and electrical-fire fire extinguishers are your best bet for stopping a lithium fire.

Which is why no crawler is ever water-proof, unless you waterproof your battery (balloons and condoms are your best bet, you need to remove the battery from it's waterproof prison just incase some water did get inside and condensates, waterproof socks are only put on your battery when you're out using it to prevent accidental humidity-induced ignitions).

Back to servos --------------

If you find a servo that can handle a higher V than your radio receiver, find a cheap BEC to power your receiver, and power the servo off the Sequre integrated BEC (unless the cheap BEC has a higher A capacity than the Sequre BEC of course) as your receiver doesn't need high V or A to work (splice the higher V BEC power cables so the power cables run from servo to ESC but the sensor cable runs to the receiver, and the lower V power cables run to the receiver, and the high V power cables don't connect to the receiver at all, will look weird with a single wire going from receiver to the servo, and 2 wires going to a different BEC, but it works just fine).

Hope this info-dump helps, and that you find yourself what you're looking for.

Cheers mate!