Two regiments are funded under the FSP as it is right.
This was confirmed by MAJGEN Jeremy King recently in a Q&A with DTR (subscription required but worth the read). In it he says there may be roughly a doubling in the number of vehicles ordered as part of the second order, or just under. So roughly 60 guns and 30 ammo carriers, assuming it is fully doubled.
Assuming 6 gun batteries (we use 4 with the m777s now), then that amounts to 10 batteries worth. Two regiments will likely use 6 of those, 53 Battery will probably use another for 7, and a small number will probably be giving to the School of Artillery to train new members on. The additional two and a half batteries should be enough for maintenance pools, attrition stock and additional training (drivers/crewies, technical maintenance training, etc).
Drones haven't necessarily been the biggest killer of armour in Ukraine. Heavy artillery has also played a major role, as has poor tactics and preparedness from either particular side. Drones are certainly a threat (and I don't believe they've been swarmed yet in Ukraine?), though often they are used as spotters rather than as "droppers."
I think the two regiments is fine so as long as they are well prepared and able to do well at what they are designed for. Keeping in mind they will be part of a wider fires system that will include a HIMARS battery/regiment, perhaps a pool of current M777 batteries, and that which is provided by aviation (the Apaches and CAG).
Applying lessons from Ukraine will be key I think. Whether we will need to ever fight like Ukraine in some areas though is probably a better question.