Its talking about the total, the IOC and FOC squadron are composed 20 aircraft (16 single seaters and 4 trainers)...we dont really know how the 83 Mk.1A are going to be "ordered" once inducted in the IAF since its not an even number, its a bit over 4 squadrons of 16 aircraft each, and a bit under 5 squadrons. Its possible its going to be 4 and a few more in cases of loss or rotations.
Also, they ordered less trainers (10 in total, meaning, 2 or 3 per squadron of Mk.1A)
My guess, this will be the squadron composition of all Tejas squadrons once all of them are delivered--->
No. 45 -> 16 Single seaters IOC + 3 Twin seaters
No. 18 -> 16 Single seaters FOC + 3 Twin seaters
3rd squadron -> 19 Single seater Mk1A with 2052 + 3 Twin seaters
4th, 5th & 6th squadrons each -> 18 Single seater Mk1A with Uttam + 3 Twin seaters...
Total 105 single seaters + 18 Twin seaters = 123 jets
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Standard squadron composition of IAF is 16 fighters + 2 Trainers + reserves...
So, overall we have 9 single seat and 6 Twin seat reserves for all 6 squadrons...