Most common is in odd figure like 11 , 21 , 31 , 41 ......and includes your shaadi and valima dress also (if your inlaws are giving it to u !) like 9 normal wear dresses and 2 bridals of valima & shaadi.
There is no hard and fast rule of 11 , 21 or 31 it can 15 ...it can be any other odd no too.
yes
it should b odd,,usually considered lucky
i think minimum 15 to 41 are normal