I'm sure they are very shorthanded right now, so bulk orders may be impossible, but when things return to normal I believe that emailing Robin is the preferred method for getting a quote on larger orders.
Hopefully we'll have another Teensy 4.x product sometime next year. But we're not going to call it version 5 until an upgrade to a different processor core. Cortex M55 looks like the most likely future. But since ARM announced it only months ago, we're probably looking at years until NXP and ST make actual M55 chips.
For perspective, ARM announced Cortex M7 in late 2014. The first round of M7 chips from Atmel, ST & Freescale (now NXP) appeared a couple years later, but with lackluster performance, high power consumption, and expensive prices. It took about 4 years from ARM's announcement until NXP & ST made really worthwhile M7 microcontrollers, and then another year until we started beta testing Teensy 4.