What a lot of questions! I hope the answers will help others as well.
Loads of young can hatch from one mating pair, plenty for several meals once they’ve grown up.
To leave them while you’re away:
For the snails to be active they must be kept moist. When it’s too dry for them they aestivate, which is like hibernation but in summer. So keep them in shade, with plenty of water to keep the environment moist. Growing food plants will last them longer. They like grass.
They can dig themselves down into the ground, that’s where they lay their eggs. I'm not sure how deep, but pushing some slate, sheet metal or glass down a foot deep would surely keep them in. I believe the French dig them a pit, which is cool and moist.
In winter outdoors in the UK they hibernate. I let them do that. If you want to them keep active and feeding they must be kept warmer.
I hope this helps, good luck!
