We (IAF alone) need 200 plus in the next five years or at most a decade. I would start with that figure and not worry about thousands. Cars are made in hundreds of thousands or millions. Most fighters are made in 100s - a few hit thousands
The problem is NOT capacity constraints or assembly line constraints or this and that. Those can be fixed easily. It just takes a mindset to de-bottlneck the production line and the supply chain and you can enhance capacity .
The key constraint is the IAF mindset. They simply have NEVER inducted a fighter or anything and gone through block modernization to get to full capability. The IAF model is to get fully developed fighter and then have it license produced and "indigenise" as much as possible.
This results in an LCA kind of development impasse, where orders come in trickle, where you cant get flying hours and initial validation and feedback to debug and plug back into the development cycle, and then there is always a moving goal post. In short the IAF does not have a product strategy, and if they do , it is plainly idiotic.
This is NEVER the way , ANY aircraft is developed anywhere in the world, from the East to West, to either military or civil. The intial versions of F16 , F18, Gripen, Eurofigther, Rafale Mig 29, SU-27 , F35 etc started with A/B models , went through block upgrades and in service debugging etc, and future iterations rolled out.
Case in point F35, the LOW RATE INTIAL PRODUCTION of F35 has delivered a total of 162 airframes! The full production rate will be hit only in 2019 . The F35 is NOT YET fully qualified for service or in IAF parlance, doesnt have "FOC"! .
IAF got a fully developed Rafale (Close to 20 years after it first flew I think), similar timeline with Gripen and all the MRCA contenders, where the versions which duked it out was close to some 3 or 4 block upgrades from inital version.
Coming back to the Tejas , what we are deploying is the equivalent of a Gripen C/D and what they "claim" they want is the Gripen NG equivalent , and all this without deploying the initial versions of Tejas.
Like the global industry, the IAF could have ordered some Gripen A/B equivalent some 5 to 10 years ago, gone for a block upgrade to C/D equivalent by now in the new build and as a Midlife Upgrade, and then gone to a Gripen NG equivalent in 5 years from now!
But no, they will order a piddly amount of 40 airframces in C/D, then want a massive avionics upgrade equivalent to a Gripen NG for the next 80 (within 4 years), and then to shore up numbers, "import" an equivalent number of Gripen NG / F16 Blk 70 and call it "indigenous" eventually, becuase it was screwed together here in India.