Yes $36 is a good "base number". This is the season so to speak.. my daughter is having all her spring Saturdays taken up by bar/bat mitzvahs.
Remember if you go to the service, no bare shoulders on dresses. If you have a son, he should wear one of the provided yarmulke (sp?) to cover his head during the service. According to my daughter the prayer books are "backwards" so it isn't the usual left to right.
I realized you were talking about adults, one of my daughters invites had sort of a "temple rules" etiquette inserted in the invitation.. It said no bare shoulders and modest dress for women, yalmulkes for young men and prayer shawls for men (not sure what a non Jewish grown men would wear!)
Check to see if it is black tie. It may be. My daughter has been invited to two black tie ones and we saw parents/guests in tuxedoes and gowns at the temple and at the party site.
If you are two adults going, MINIMUM of a multiple of 18 x 2 guests. For example one time both of my kids were invited so I gave 36 x 2= 72.