The S-400 may be a mobile version, but it's not the S-300V which comes on tank tracks. And the S-400 comes under the IAF, so it will be used for base defence, and it will be moved around every few hours over a sufficiently large area to keep the enemy guessing.
We only have to park them in and around air bases along the Pak border.
Let's not forget that the IAF actually wanted 12 of them, but have settled for 5 right now. It's possible DRDO will provide the requirement for the remaining 7 systems using indigenous tech.
Red = What I think will happen. Srinagar, Sirsa, Jailsalmer, Bhuj and Hasimara.
Blue = The holes in our defences that require something like the S-400. Bareilly, Gorakhpur, Chabua and Kharagpur.
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I won't be surprised if we position all our S-400 near the Pak border while we depend on the XRSAM for the China border.
What's important to note is the army will be operating under the cover of the S-400. So even if we say base defence, it's basically wide area defence. The S-400s do not have to leave the vicinity of the air bases.