Hi Ben. The DMX-512 standard specifies no more than 32 fixtures per daisy chain. This is a somewhat subjective number though, as nothing magical happens if you add fixture 33.
What’s going on is that every time you add a fixture or junction you’re causing a point of signal degradation and reflection. After a certain amount of this, the aggregate signal you’re sending weakens to the point that glitchy or erratic behavior begins to occur.
In practice, once I get around 24 fixtures or 1000ft of combined cable on a single run I add a splitter. This must be a DMX splitter (which is active and optoisolated) and not a Y-cable. Coming out of the splitter the fixture count and cable length both start again from 0. This caveat aside, there’s no limit to the number of fixtures you can control with a light board, though you’re still restricted to 512 channels per universe. Hope this helps!
-Jeff