Different question: For each ship class, how many synths are required? Based on the prior conversations, it would appear that Fighter-class hulls need only a single synth. We know that Cruiser-class hulls need four synths. How about Capital-class hulls? Or, much more relevantly, how about Frigate-class hulls?
If I had to take a stab at it, I would assume that following minimums are used:
Fighter: 0 synths. Rather, a software-only equivalent is included to manage the fighter's systems.
Shuttle and Free Merchant: 1 synth (Delta, the generalist)
Other Frigates and Bulk Freighter: 3 synths (Alpha, Beta, Gamma)
Other Cruisers: 4 synths (Alpha, Beta, Gamma, Delta)
Capital: 8 synths (two of each)
These are the minimums, not necessarily the recommendation.
Fighters are screwed as they are just cockpits. I would probably guess that fighters with spike drives are virtually unheard of. Any such fighter has to include cryostatis functionality in the cockpit and the synth functionality is purely software. If something goes wrong, the pilot is basically screwed. Almost all fighters only use system drives, and if they have to transit between systems, they are carried.
Shuttles and Free Merchants are equipped to use a single Delta synth. That means the synth is busy the whole time, but the ships are small enough that it is manageable. Most shuttles only use system drives, but shuttles with spike drives are not nearly as rare as fighters. No shuttle is equipped to use more than one synth, but it could be done. Free Merchants are more likely to be encountered using two (D, D), three (A, B, G), and rarely a full set of four, but often only a single Delta is used.
Frigates (other than the Free Merchant) and the Bulk Freighter can get away with using only three synths, foregoing the Delta. However, this is fairly uncommon as, given the price of the ships, adding in the Delta for redundancy isn't enough to quibble over. The stated minimum standard, however, is still just three synths for these ships. (Obviously, the Celestial Voyager is using the more safe configuration of four synths.)
Other cruisers require the full compliment of four synths. In fact, some of the larger frigates will use more. Perhaps they add in an extra Delta or two. Perhaps they double the Alpha, Beta, and Gamma, and use a single Delta. Or maybe they just have a double set for a total of eight. Regardless, the minimum is four, and the top is rarely, if ever, more than eight.
Capital ships are just too large to not have a double set of synths. It is not uncommon for them to ten synths (two extra Deltas) or twelve synths (three full sets). The minimum, however is a full eight synths.