My recollection of CA state hiker-biker sites is that they don't have a max allowed number, ie they don't fill up. This is logical since you can't reserve a spot, there's no way for the park system to know how many people are touring through, and camping anywhere outside the campgrounds is generally illegal in Big Sur.
On busy weekends it might be cramped. But I recall from staying at the H/B in Pfeiffer Big Sur that there's room for maybe 25 tents so I wouldn't worry