A brick with a 2" by 8" face, surrounded by 1/4" of mortar will need to be replicated 776 times to have an area of 100 ft^2. The number of bricks actually required depends on their size, the edge finishing, the mortar thickness, breakage, the nature of the cuts, and probably several other factors, including the number of bricks thick the wall is.

Brick retaining walls are usually 2 walls with a brick row on top. ... Your wall 25 x 3 = 75 x 2 (the back wall) = 150 sq feel x (how many of your brick per sq foot) + 100 bricks for top row = total brick x 10% waste factor

This question's answer is dependent on the size of the brick. There are two variables; the length of the brick (L); and the height of the brick (H). Since it is known that L * H = A (area in units squared), the formula (L * H = A) can be easily found if two of the variables are known. L * H =100 can be described if H = 10 thusly:L * 10 = 100, => L= 100/10 => L = 10. So: 10 * 10 = 100 (yes it does).This can be used regardless of what L or H is equal to. If only the 100 is known, no answer can be given because there is no known quantity for L or H and it can only be guessed, which has nothing to do with math. There is a possible third variable that is the thickness of the mortor used to hold the bricks together as proposed in the answer preceding this. It is easily remedied with the addition of the variable to the formula L * H =A. One must simply modify the formula to include the thickness of the mortor line (M):(L+M) * (H+M) =A If the line thickness is different for H or L, simply add another variable (N) to the equation (L+M) * (H+N) = 100. Where M equal thickness A, and N equals thickness B. Put in your real numbers and follow the PPMDAS rules of math (Google it) and you will get the right answer 100% of the time. Powers, Paretheses, Multiplication, Division, Addition, Subtraction (PPMDAS)

It would depend on how big the bricks are that you plan on using. I'm sure some of them vary in size. You should pick out the bricks you want to use first. From there you can measure them and then calculate approximately how many you will need.