What are the information available. If you know entire time of the colony from start to date. You can have a rough estimation by dividing that time from generation time. Example lets assume colony started T time ago. if the generation time is t. T/t gives you the number of generations in the colony. If this is not the case more information needed to give a proper answer.

Let us say, there are 10 insects that I inoculated to certain plant host. There are 14 offsprings that emerged after 25 days. Then the next observation is really hard for me to count since all of stages in the life cycle of the insect are present. Can I solve the number of generations by this data? Thank you for your response sir.

instars can often be seen in altered body proportions, colors, patterns, changes in the number of body segments or head width. After moulting, i.e. shedding their exoskeleton, the juvenile arthropods continue in their life cycle until they either pupate or moult again. This period of growth, instar, is fixed, however in some insects, like the salvinia stem-borer moth, the number of instars depends on early larval nutrition.

You have to basic idea regarding morphological characters of each instar. By that you can judge the no. Of instar attained by particular insect.