What Does The Mounting Gasket Need To Cover?
Here is an example of a 9000 series AFB carburetor base, which illustrates what needs to be covered.
The primary and secondary throttle valves are open to each other, so the gasket can be open in the middle, or have 2 holes, or 4. Doesn't matter which.
The outlined area needs to be covered. This will seal the required area from outside area.
In this example there are 4 mounting holes. The mounting gasket (flange gasket) can use any of the 4 holes.
In this example (A) vacuum is fed from the bore to these points and travels through the cut out and up into the carburetor. The mounting gasket can cover these (B) cut outs.
Other cutouts (C) are simply casting holes and do not need to be covered. There is no vacuum fed in these (C) places. There are several on this mounting. You need to determine which ones actually feed up through the carburetor. Blowing air through is one way.
As far as vacuum leaks, the only place you have to be concerned with is exposing the outside air to the outside perimeter of the flange.
The gasket on the right is correct for this flange. It covers all of the cutouts, but the vacuum cutouts are fed through the bore, so that is OK.