If you don’t know the exact numbers, but you can make a good, educated guess, you can give a:
ballpark figure = Schätzwert
A ballpark is another name for a baseball stadium. Exactly why an estimated number is called a „ballpark figure“ is not clear. One (quite logical) theory claims that sports commentators would estimate the number of people in attendance at baseball games, and this then gave us the phrase. Another (more interesting, but maybe less logical) theory is that the expression was invented in the 1950s, during the early years of the space race. As it was not an absolutely exact science, when a spacecraft returned to earth it was said to be „in the ballpark“ if it landed within an acceptable area.