You can install one for the G5 and one each of your GDUs.
My G5 uses the GA26C on the glare shield. The unit needs a GPS reference to keep attitude info accurate.
The 650xi needs a Garmin GA 34, GA 35 or GA 36 unless you want Comant. Otherwise you have to find and check the relevant TSOs. TNC. TSO C144. I think that is a WAAS spec.
GDU’S use GA 55 or GA 56. BNC. TSO C129 CLASS A-1. There is a discrepancy in the G3X install manual. In edition AU, it is stated not to use GA3X WAAS antennas with G3X GDUs; but, in Section 20, the same manual states it is fine to do so and also provides some other antenna options. I used the GA 56 in my install.
After you think about it, I think the only question is do you want a GPS antenna on each of your GDUs or just one of them. One way to determine your comfort level is to wheel your build outside and see what happens when you start failing units.
One more thing is not to forget to create loss in your signal. I think the GTN xi and the GDUs both need some loss in the signal which you can create by having sufficient cable length. I used RG 400 and cut 8 foot lengths for each antenna connection.
Warm regards,