There is no limit to the number of trees you can buy on Farmville in one day. The limitation is the amount of money you have to spend on the trees in the game. Another limitation is the size of your farm. You can only buy as many trees as will fit on your farm. All of the trees have different lengths of time it takes for them to grow fruit. Some it is just a day, others it is up to four or five days. The sale of the fruit is usually significantly less than the cost of the tree, but unlike crops, the trees continue to bear fruit repeatedly so you do not have to purchase them again. This means it tends to take at least ten if not more harvests before you even break even on a fruit tree. But then for the rest of the game you will be making profit.

Of course, unlike crops which turn a profit as soon as you complete the process the trees take usually a full month before you are making Farmville money on them. But the advantage of trees, other than their continued production of fruit, is that the fruit does not wither. You can leave the fruit for an endless amount of time and still harvest it, so you never lose money on a tree.