An orbit has 6 degrees of freedom, observing the direction of an object has two parameters. That means you need at least three observations to have any sort of chance to determine an orbit. If these three observations are close together then they are largely redundant - the object will move approximately in a straight line so you can predict one measurement from the others and you only have two useful measurements. If these observations are too far apart, it's not clear if they are all the same object.
Typically discoveries are based on two or more observations close together for a very rough initial orbit estimate, and then more measurements at larger time differences to determine the parameters more precisely. Ideally this orbit lets you search for the object in older images to increase the time span between first and last observations as much as possible.