Hi,
The dose of botox should be tailored to the desired response while factoring safety and the avoidance of unwanted side-effects such as droopiness, excessive muscle paralysis rather than the treatment sessional number.
In my experience, fewer units of botox are required for some patients after repeated treatments as the original muscles treated decrease in bulk and strength.
Your preferred aesthetic doctor will be able to assess your response from previous treatments and advise if higher doses are needed to optimize your clinical results.