BoxRec

In the USA bouts are 3x3 min rounds

how many miles and what pace should a amateur boxer run. Would Sprint training be more effective for the Ams?

No expert, but know a bit about training in general.

Do mostly low intensity jogging for cardio. Pace depends on your form ofc.

Use a pulse clock that alarms when under 120 or over 140 BPM. Stay closer to 120 than 140.

You can mix it up with a few long interval training here and there. You can start with shorter runs/breaks. Check out the 4X4 method.

In short:
4 minutes of 4 periods running.
Shorter breaks 2-3 mins.
High intensity throughout

It takes a bit of practise to pace yourself for this routine.

Good luck!

Only recently aged 49 have I discovered that fast excersise bike sessions have really given me an extra couple of gears in my running .
Try 15 min fast sessions incorporated into your usual workout.Some days these can replace your runs as well.

Warm up with slower cycling for a couple of minutes then a quick pace you can keep up for 15 mins or more but which makes you work

One thing I found that was when I was fighting in the ams... they were 2min rounds then was running 600m in under 2 min. With 3 min rounds you could probably do 8-900M (So like half a mile in U.S.A. terms). It's important it is a distance where you have to push yourself hard to get under the 3min barrier and do one for each round.
I don't believe you need long distance running for amateur fights, they're short, sharp bursts. If it was me I would rotate between the half mile round runs and 100M sprints working my way up to 10x 100M sprints.
Essentially you need to be able to explode and fight at a fast pace for 9mins long distance running will help with that but IMO the sprints & sub 3min runs.
Ali used to run 6 miles in 40 mins while wearing army boots. 40 mins is almost a 15 round fight, Ali used to dance for 15 rounds. The long distance running was certainly needed so his legs would have the stamina required for a 15 round fight.