Dear Jesai,
Your question seems very vague to me. It is not clear whether your question pertains to the situation wherein the criterion of the number of employees employed for applicability of the Acts becomes less subsequently. To my knowledge, not all Acts have such a criterion for application. Most of the labor laws whose applicability to any establishment is once determined by the total no. of employees employed continue to be applicable not withstanding the fact that the number gets reduced later. However, only if the application of certain special provisions of the Act is necessitated by certain minimum no of employees on roll, the subsequent reduction in that number renders those special provisions only inapplicable.
Therefore, the simple answer to your query is that what all Acts applied to your establishment when it had a strength of 100 or more employees are still applicable even when it falls below hundred.