Speed for any vehicle can be calculated with theoretical numbers but testing with your vehicle is the best way to actually know. Long story short there are lots of sources of force acting against the vehicle especially once you start driving it, there is friction with the ground or bearings and air friction or drag which increases with the square of velocity and depends on the surface area hitting the air. Due to aerodynamic forces the friction on the bearings etc may be variable since the mass of the vehicle is fixed there is a constant down force (gravity) but anything that creates lift will reduce this force and anything that creates down force will increase this force.
Ecalc.ch is a site with calcs to help get some in theory numbers crunched but ultimately going to have to just build and test since this is a one off thing.
I built an esk8 along with being into quads the esk8 forum has tons of knowledgeable folks with regard to those motors, gearing/setups, but I think air powered car is pretty unique project, think I only know one YouTube channel that did it... Pretty sure guy's name is Ivan does a lot of massive custom 3D print stuff.
Regarding many small motors vs one big, bigger motors and speed controllers to get power delivered and control the speed cost quite a bit more (as do props). In general a bigger blade has more mass and so is harder to accelerate and decelerate quickly but also the size of the disc the blade sweeps through tells you how much air it hits (also the pitch or tilt of the prop tells you how much air it "scoops" each turn but less scooping means a cleaner "cut" through the air, low pitch will be higher efficiency but less thrust).
I made this rig to test out various motors ESCs and props so can test out and know roughly the lift capacity of my hexacopter and just for giggles
it shows the weight being pushed onto some load cells:
Really would be safer to clamp this down and operate it remotely but I had a pretty good grip on it
