If we have a plate that has 2500 colonies on it, can we use this plate to calculate cfu's

No, the solution with bacteria in it needs to be diluted in order to be in the range of 30 – 300.

A colony-forming unit –CFU is used to estimate the number of viable (ability to divide by binary fission) microbes in a sample.  

A sample of microbes is diluted and spread on a plate. Then, the number of colonies is counted, assuming that each colony have grown from a single colony-forming unit, or CFU.

Plates with more than 300 colonies are difficult to count while those with less than 30 colonies give statistically unreliable numbers of colonies to count.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.