Re: putting a fueling station into orbit
"Q: how many additional boosters can you strap onto a Falcon Heavy before it can't handle the load?"
It's not a question of whether the vehicle can take it; the ground infrastructure (pads, towers, etc.) is not set up for anything more than two strap-ons. *
Unless something seriously bad happens to the BFR plans, Falcon Super Heavy probably won't ever exist.
* apparently - I asked Elon on twitter, unsurprisingly he didn't reply, but a random someone else did - so treat the information accordingly