There are a number of factors which determine the spacing of signals on a route. Among them is the number of trains, and in the case of subways and elevated lines, it's a matter of overlap too so that the trains can travel closer together.
This all plays into a number of other things such as lead time between trains and the length of trains which will determine the block spacing, and all this works with the speed at which they will travel. The general rule is the blocks are spaced to hold the longest train that will run on that section of track.
Some newer subway systems have some pretty sophisticated technology now associated with their signaling systems with some employing sensors that will automatically adjust the speed of a train to match the one ahead so that the trains can continue moving with minimal stopping outside of the stations as required.