If the code you are debugging is executing out of write-able memory, then your debugger can support an effectively infinite list of breakpoints by writing some instruction that causes an exception to be thrown anywhere you want a break, then handling that exception by looking up the faulting instruction address in your breakpoint list.
Most processors also give you hardware data access breakpoints as well. These typically sit on the CPU's data bus interface and can fire when the address or data that's about to hit the bus match the respective breakpoint registers. There is usually an option to only trap reads, writes, or both. Sometimes you get interesting things like a mask register that lets you trap on a whole block of memory.
One of the most interesting hardware debug features of modern processors is the branch trace which keeps a running list of the last N branch instructions that lets you reconstruct a "how did we get here" story.