SpaceX has just announced their new smallsat rideshare program. Is it known how many satellites can be launched at once? Any estimates?
From that same page:
Those are ESPA rings, for EELV Secondary Payload Adapter.
The initial ESPA ring was designed to support a 15,000-pound (6,800 kg) primary payload and up to six 400-pound (180 kg) secondary payloads. Each secondary spacecraft is mounted radially on a 15 inches (380 mm) diameter port and is allocated 24 inches (610 mm) × 28 inches (710 mm) × 38 inches (970 mm) volume. This has led to the colloquial designation of ESPA-class payloads.
One spot on the ring can be occupied by a small satellite, or by a cubesat dispenser.
the image also shows a different ring:
ESPA Grande - The "stretch" version of the ESPA Ring, accommodates four 300-kg secondary spacecraft on 24-inch ports.
The rings can be stacked as required.
The F9 User Manual has not yet been updated, but that's the place where I expect more details on what combinations are possible, will appear.
As for an estimate: there are a bunch of variables that would lead to different estimates. The payload is either mass-limited or volume-limited depending on satellite mass, etc.