Your question ("How many muscles are affected by giving botox injections for one unit?") does 'not' make any sense - please clarify/ rephrase, Satyaprasad. 

With regard to how many muscles being affected, that depends on the location of the injection (i.e. place of toxin delivery). 

There is 'no' formula re. magnitude of wrinkles and amount of toxin (BoNT) required. This is because there are numerous confounding factors. The experience of the doctor, the patient's previous response to the toxin (as determined by muscle mass, local anatomy, et.c.), skin quality, and the depth of toxin delivery, are some of the key factors that affect the amount of BoNT required. 

Thank you Dr Niroshan. I will take your advise of rephrasing. I do admit that understanding botox action was necessary before posting the question. I thank you for making me go back to my basics.

No probs, Satyaprasad. As Lee Seng has mentioned, there are different types of BoNT that are available. The size of the molecules varies, e.g. Galderma's type (Dysport) has larger molecules than Allergan's (Botox), due to the supporting proteins. Merz's type (Xeomin) has no traditional stabilising complexes and, therefore, theoretically, is less likely to cause an allergic reaction and is less heat-sensitive.

Again, theoretically, the heavier molecules are more likely to spread with gravitational descent, and this could affect the size of the treated area.

If you have any more questions, I shall be happy to share my thoughts!

These effects principally depend on the muscle anatomy (region of the injection). However, in my experiences frequently used and strong muscles are required larger doses than smaller ones.