Is it permissible to perform ghusl for janabah with a small amount of water, and is there a specific limit to the small amount of water used for ghusl for janabah?

There is no specific determined amount of water required for ghusl (ritual bath); rather, it varies according to people's circumstances. What is obligatory is to ensure water covers the entire body without extravagance. The Prophet, peace and blessings be upon him, used to perform wudu' with a mudd (a measure of volume) and ghusl with a sa' (a larger measure of volume). This specification does not mean it is obligatory to adhere to it; rather, it is understood to be a recommendation according to the majority of scholars.