How it realated with T90?
SPHs are associated with armoured brigades. Each T-90 armoured brigade consisting of 2 T-90 regiments with 118 tanks requires 1 SPH regiment of 20 guns. Some brigades have 118 and some have 135.
So we have 5 brigades connected to SCs and up to 10 brigades connected to RAPIDs, along with 7 independent brigades. So these are currently a mix of Arjuns, T-72s and T-90s. More brigades are coming up for the Chinese border. So, 22 armoured brigades need 22 regiments of SPH. 300 K9s will give us 15 regiments.
The two Arjun brigades will have 2 regiments of Catapults, which are basically towed guns placed on the Arjun's hull. So the K9s will be spread between T-72s and T-90s. But in time the T-72s will be replaced by FRCV, which will come with its own SPH, so the K9s will have to be transferred to T-90 armoured brigades in the future. But we won't have 15 T-90 armoured brigades for the 15 K9 regiments with the current number of T-90s. If K9s are linked to FRCV instead, then it will defeat the purpose of the FRCV being a UCP, so that can't be done.
We are very likely to go for 450+ more T-90s though, since our license goes up to 2028 whereas we have orders only up to 2024. And if FRCV is delayed, we will need another 450+ T-90s. This will take our T-90 numbers to about 2500. At least 1770 tanks are needed to make 15 armoured brigades at the minimum.
We ultimately want 49 armoured brigades at the highest in different configurations, but that needs to be met by FRCV in the future.