To match up a bearing you can normally simply read the 4 digit number from your old bearing, e.g. 6003, 6203 and simply select this number from the dropdown box for which bearing you want to order.
Some bearings (e.g. gearbox bearings normally sit in oil so you would pop out the plastic shields on the side of the new bearing before fitting. These plastic shields keep grease inside the bearing (not needed where the bearing sits in oil)

no, all these bearings are sold individually