I'd say 2 guys are good,three even better!--.even if the third guy just watches,he can always call 9-1-1 if something does wrong!..this CAN be very hazardous,BE CAREFUL..not trying to scare you,but even a riding mower weighing 300 lbs can cause some life threatening injury in a heartbeat if it gets away from you!..
If you can back up to a hill or wall,its a lot easier!--the less angle on the ramps,the better...
In my younger days I have gotten quite a few riding mowers loaded into my pickup bed alone,but only ones without a deck,and if they had no engine, that was even better --less weight!..and I'm not a big guy really,I'm 6'5" tall,but only weighed about 150 lbs then!..I used my legs mostly,they are a lot stronger than your back or arms are!..but I knew better than to attempt loading a "Garden Tractor" alone,and if it had wheel weights,forget it,even two guys would have a struggle loading one with them on it,and you'll need BEEFY ramps too..
To load one alone,I'd lift the front end up and put the front wheels on the tailgate,block the rear wheel with a chock so it wont roll backwards,then get behind it and hoist the rear end up,while shoving it in at the same time!..it was often a painful experience though,and my back would ache for a week or more after doing that..but it was often a "take it now or it will be gone" deal,so if I wanted them, I had to get them loaded alone,or lose out!..
I had too many tractors spit the ramps out if I atttempted to drive it up onto the truck,I got hurt twice doing that,and cracked a differential in two also when one fell,so I never try that method any more--plus,your weight AND the tractor could crack a wooden ramp in two halfway up as well..
I prefered a 4x8 sheet of 3/4" plywood stiffened with 2x4s on edge as the ramp.rather than just two planks,it reduces the chance of injury a lot--but is a real pain to lift and store in the truck bed after the tractor is in it!..
I have put a few in low gear and let it "idle" up the ramps as I guided it with the steering wheel,but most tractors with safety switches wont let it run without you sitting on it,and it's probably not the safest method there is either..and wont work if it wont run!..
I'd highly reccomend getting a small winch,either a hand cranked "boat trailer" or electric one,or at least a "Come-A-Long"..even with two guys,I've had a few tractors get halfway up the ramps ,than go off course and one tire slips off the ramps,or the ramp itself slides out from under it altogether!..then you either drop the tractor,or have to struggle to get it lined up on the ramps or the ramp back under it again,even with two guys it can be difficult..
I lost count how many times my "helper" would pinch a finger or something,and LET GO when we had it halfway up the ramps,and I'd have to push it in the rest of the way alone!---and I have been chased by "runaway" tractors as they rolled backwards when I attempted loading them alone using ramps,I often could only get them halfway up!..with a winch or Come-A-Long,you have complete control,and can stand away from it,while IT does all the work,the only thing to watch out for is the tires dont turn on you and fall off the ramp halfway up..you can put a bungy cord on the steering wheel to help keep the wheels straight ahead..
I used a Dodge Caravan to move 3 tractors recently,and I will say this,I like the much lower height a lot better !!--a trailer is good too,but loading a tractor with 8' ramps into a pickup bed 36" or higher off the ground (as is my 4x4 GMC),it can be quite a challenge..especially when the deck bottoms out as it gets halfway in!..that's when the winch or Come-A-Long is nice,it'll hold it there, while you lift up on it and "unsnag" it..curved ramps prevent that better than flat planks too...
I might make a tilting "table" that will go in my reciever hitch to carry tractors on,that way I can drive them right up on it and pin it in place,and go--and still have my truck bed free to haul other items..as I get older,lifting and pushing become harder to do without injury--work smarter,not harder,right?..