The Apache isn't an interim measure until the LCH comes online. The Apache will form part of the high-low mix of which it is the high component. The Apache is far, FAR more capable than the LCH hence having plenty of these platforms is a real boost. And I have no doubt 70-80 units of AH-64D Blk.III are certain.
The advantages of the Apache are avionics mainly, which can be upgraded for LCH in later stages. In terms of weapons or flight performance it isn't far more capable, people just hype the differences between combat helicopters, because they think it's similar to fighters in different classes, but that is not the case!
Apache D Longbow vs. LCH
Gun: 30mm vs. 20mm
ATGMs: 16 vs. 12 to 16 (with the latest changes of the stubwings)
Rockets: 76 (4 x 19) vs. 48 (4 x 12 with the MBDA pods)
AAMs: 8 (4 x 2) vs. 8 (4 x 2)
Range: 476 km vs. 550 km
Speed: 265 km/h vs. 275 km/h
Ceilling: 6400 m vs. 6500 m
Sources:
http://4.bp.blogspot.com/_Dc2Wx4jR9...ABN0/iWSE6dxGWi0/s1600/CIMG2774.JPG]LCH+specs
Boeing: AH-64 Apache Specifications
Keep in mind that the US offers the same Sidewinder, Hellfire or Rockets for LCH, that the Apache uses too, which means the weapons itself don't make a big difference and neither does the weapon carrying capability. So for normal fire support of troops or armoured vehicles, LCH will be more than useful too, with the main differences in the gun only, while it is similarly capable in anti tank or A2A capability.
More interesting will be the latest radar and avionics features of the Apcahe for IAF, if we can customize the Apache with some Indian or Israeli techs (satcom and datalinks), they would be great for forwarded operations, be it recon linked with UAVs and AWACS, low level SEAD or pre-emptive attacks without the need of ground forces. That's possibly what IAF might have in mind with them, to have a low level attack capability, besides it's fighters. Escorts for transport helicopters, or fire supports for ground forces won't be their priority anymore, when IA airwing and IAF will have LCH for these roles.
You really need to think about what the forces might want to do with these helicopters and what alternatives they have before get into conclusions, then you will see that your numbers are way too high again and that the requirements of IAFs combat helicopters are changing.