Personally I've only ever needed more than one SRV when I was doing Guardian ruins.
That was before I learned the trick where you park your ship really close to the ruin so your PDTs can shoot down the rockets Guardians are shooting at you.
I do tend to put C4 hangar on my exploration ships when I'm planning to be outside longer in a bigger ship, but firstly, I've never actually needed it - I always got myself out of all trouble I've encountered, particuarly deep canyons and rough terrains the ship can't land in, and secondly, nowadays I mostly explore in DBX so C4 hangar is out of the equation anyway.
Plus I think having a spare SRV is detremental to fun, out there. If I don't have one, I have to think where I'm going. One time, I've spent almost whole two days in a canyon I've jumped into and couldn't get out of and which was too rough and narrow for my ship. I ended up driving about 60 km through it until it opened up enough for me to get out and start looking for a landing spot for my ship.
If I had a spare SRV, I'd be tempted to simply self-destruct and get on with things. But this way I had to fight my way out of the situation and it felt awesome.
Though sometimes I go out exploring in my Cutter and then I make a proper mothership out of her. Big SRV hangar with a lot of SRVs, big SLF hangar and two SLFs (imperial and guardian, usually because they're most fun) and I go crazy. Racing around, smacking stuff up,...
