Correct - number of breaker spaces has no bearing on system ampacity. They are two unrelated parameters, like money in your account vs checks in your checkbook.
Every appliance draws the current it needs, and no more. Varying as its needs change. The sum total of those actual draws (for each phase) constitutes the actual load on your panel. Having more breakers in your panel doesn't add any load, the loads do.
Around here, we advocate a gross excess of breaker spaces, since they're dirt cheap. But your whole 150A ampacity could be consumed in 4 spaces. I had a friend with 400A service (two 200A panels). 2 breakers (4 spaces) accounted for 140A of emergency heat on a heat pump. But only when the emergency heat was on, and that means the heat pump proper is not on, so the 16A heat pump can overlap into that same 140A. So it isn't 140A+16A, it's 0A with nothing running, 16A with heat pump running, OR 140A with emergency heat running.
If you overload your panel you will get a main breaker trip. That breaker is there to protect the panel and service wiring from damage from overloading.
No professional electrician will build a panel that will overload. This is determined by a formal process (2 actually) called a Load Calculation. You input square footage of house (as a catch-all for general lighting and receptacles), an allocation for 20A kitchen and bathroom circuits, the nameplate data of most 240V appliances except some wild math on the range, and you add them up. It must be less than the service ampacity or they won't give you an occupancy permit.
That is vanishingly unlikely to trip the main breaker. Back in the dark days, they permitted house with up to six "main breakers" which added up to well over the service ampacity, hanging their hat entirely on the load calc.
The Achilles' heel of this system is upgrades. People add loads and don't redo the load calc. That was particularly a problem with Rule of Six panels, which is why they are outlawed.