By default, there is a 10% chance of getting 1-2 seeds per crop harvested.
By default, crops that regrow on harvest do not drop seeds.
You can edit the config in Generic Mod Config Menu or by editing the mod's config.json file after starting the game once.
Technical
Requires SMAPI, uses Harmony.
Implements a Generic Mod Config Menu interface to change config settings in-game.
Compatible with Mod Updater for automatic updates.
Code is at https://github.com/aedenthorn/StardewValleyMods.
If you want to complain or ask for help or help me test my mods, you can visit my Discord server.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.