The short answer is “not a lot!”
Overview of the SAMD21 Arm Cortex-M0+ Based Seeed Studio XIAO is a good summary
Maximum current per I/O pin also useful (on this forum)
Assume maximum 2mA output per digital output pin and I think you are reasonably safe. Hardly enough to drive an LED and always best to connect driven devices between the power supply and collector of an NPN transistor with emitter to ground and 2K2 resistor (or higher if transistor gain is enough) from the output pin into the transistor base.
If you need a power transistor (for which the gain is typically small) use a Darlington pair.