They say it can transport about 100 tons. That's not much for a colonization effort. The Mayflower that transported the pilgrims to America was rated at about 180 tons. They could expect to live off the land for the most part whereas whoever takes the trip to Mars will be entirely dependent on what they bring with them. Without help from the natives it's likely that the Mayflower's people would not have done as well if they managed to survive at all. Maybe the Martians will help Musk's colonists.
Well, just like when Musk launched the Autopilot saying this is going to become our self-driving car he's exaggerating quite a bit what it'll do in the short term. It'll be an outpost, sustained by Earth resupplies and the bigger the outpost, the greater the need for resupplies. It'll be a very long time before you hit critical mass where each expansion would make it more self-reliant. It'll mostly be a proof of concept, can we expand the living quarters with on-site materials or do we need domes from earth? Can we generate enough food, water, air, heating and power and so on? The burden on Earth needs to go down, then the size of the outpost can go up.
I expect they'll keep enough emergency supplies and consumables in reserve to survive while they try things out and figure out what works and doesn't. But if it doesn't work, we have to send more supplies and less people or all supplies and no people or in worst case just abandon it. Though I don't really believe that, I mean if they just sit in a bunker and eat canned food like on the ISS it's hard to see any reason why they should be forced to leave. But they also wouldn't really be making any progress towards colonization that way, it'd be just survival. Then again, surviving Mars might in itself be the first step since we haven't actually done that yet either.