500-1,000 items is a good starting point if you’re including plushies with various color ranges, like toys, gumballs, stamps, books, food items, clothing, drinks, etc. However, a pet site wouldn’t necessarily need to launch with a ton of items at first. It can launch with a smaller selection, like 100-300 items, depending on how the game is set up. Expanding the game over time is a good idea, especially if you don’t have many artists to create a lot of items in the beginning. 500-1,000 items is a lot and can take a while to launch. 100-300 items is ideal for an indie developer with one or two artists.
As long as each item has a purpose, the game can still function well without a massive inventory. Each item serving a purpose is key. For example, a food item can have multiple uses, but if you have a lot of food items, then it’s fine for some to have just one use (e.g., one food point) for feeding your pet.
I also agree that 5-10 shops is a good starting point. You can always expand with more shops as the game grows, especially with specialized shops.
If you’re planning on having a battle dome, then a weapons/battle shop would be ideal, stocked with all types of weapons for players to equip their pets.