What part of the country are you in?
When it comes to fruit trees, it really is the more you have the better.
Apples - You need to be in a location that gets a good solid winter freeze. There are maybe only 4 varieties that will produce fruit if it does not freeze in your area. With apples, each plant needs to have a cross pollinator that is flowering at the same time. Semi-dwarf trees will produce about a bushel and a half of fruit a year once established (3-5 years). Standard size trees can take as long as 7-8 years to come into full production but will produce 10-12 bushels of fruit. Standard trees are also harder to control and pick fruit from.
Pears - just about everything that applies to apples applies to pears as well. Pears are probably the easiest fruit to start with as there really isn't that much that harms them (except fireblight). Pears don't really need the hard freeze like apples do.
Cherries - Cherry trees will produce a lot of fruit on one tree. They do not need a pollinator so you can plant only one if you wish.
Plums - There are two types of plums, Japanese and European. Euro plums are self fertile and do not need a pollinator. They are considered more of a cooking and prune making plum. Japanese plums need a cross pollinator and produce larger, sweeter fruit. I have one euro plum tree and this year alone I got over 36 gallons of plums.
I can't help you much with apricots as they do not grow in my area.
As I said before, with fruit trees, the more you can plant the better. Thinning (removing some fruit while it is small) is very important for plum and apple trees. The trees will set way too much fruit if left to their own devices and you will have lots of small, undesireable fruit come harvest time. If you don't thin, you also risk having branches break (happened to me this year on a neglected 30yo apple tree). Not only will you get smaller fruit and risk damaging the tree, the tree doesn't care if it sets fruit every year. The tree will put all its energy into producing seed(fruit) that it might use all its energy for seed production one year, and you might not have any fruit on the tree the next.