This is how I would go about figuring out how many sapphires to
surround an opal.
First, find the circumference of the opal - add length and width,
then divide that sum by 2. Now you have the average diameter of the
opal, call it OD.
Then add the diameter of the sapphires, SD, that you wish to use to
OD. Call this result Jewelry diameter - JD. Multiply JD by 3.15 as an
approximation of pi.
The result of the multiplication is Circumference or C.
Now divide C by SD and you will get the number of sapphires that you
need to surround the opal.
If you are going to bezel set the opal and the sapphires, include
the width of the bezel in the diameter of the stone.
This will all probably yield some fractional number like 8.67. The
whole number 8 is how many sapphires you need, the fraction portion
is how much extra space you have to divide between all of the
You still have to allow for space to set the stones. So as John
pointed out, you are going to have to do a visual trial anyway. This
bit of arithmetic just gives you a way to figure out how many of what
size you might want to try.