The amount of spots in a system where orbital projects can be deployed is limited. To represent this, each orbit has a certain amount of orbital capacity. The amount is determined by several factors, and (system wide) projects use up this capacity.
Normally, simple projects (such as the Planetary Defence Grid) take up a single orbital capacity, while system-wide projects (such as the Lagrange Point Conduit Stations) take up a point of orbital capacity in every planetary orbit. Of course, projects can have different specifics.
Each orbit, and the sun(s) itself, generate a certain amount of orbital capacity. Each planetary orbit generates a small amount of orbital capacity, which is then increased by the presence of certain celestial bodies. The following list shows which factors are relevant:
Note that a large planet that takes up the normal 2 planetary orbits will generate at least 6 orbital capacity: it takes up two orbits, so it generates 2 orbital capacity from the 'base for planetary orbits', and +4 for being a Large Planet.
The sun itself also generates an amount of orbital capacity: