I have seen a lot of ICOs, where fixed and minimum amounts are asked for. For instance, an ICO might ask for $100 as the minimum amount anyone can invest and there are some that sometimes ask for huge amount of money. This can result into investors not wanting to invest into the project, because they are being care of not investing what they cannot afford to loose.
the aim of the minimum amount is to keep investors serious in buying. because if we have spent a lot of money then we will definitely be better and more serious in trying to give good input. if the ICO does not provide a minimum limit, what happens is that investors will enter and leave themselves without being able to make meaningful contributions.