C# code for inventory system that has a limited number of slots, and the ability to stack some items (with a different maximum size stack for different items).

InventorySystem.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace BusinessObjects.RedditRPGQuestion
{
    public class InventorySystem
    {
        private const int MAXIMUM_SPACES_IN_INVENTORY = 10;

        public readonly List<InventoryRecord> InventoryRecords = new List<InventoryRecord>();

        public void AddItem(ObtainableItem item, int quantityToAdd)
        {
            while(quantityToAdd > 0)
            {
                // If an object of this item type already exists in the inventory, and has room to stack more items,
                // then add as many as we can to that stack.
                if(InventoryRecords.Exists(x => (x.InventoryItem.ID == item.ID) && (x.Quantity < item.MaximumStackableQuantity)))
                {
                    // Get the item we're going to add quantity to
                    InventoryRecord inventoryRecord = 
                        InventoryRecords.First(x => (x.InventoryItem.ID == item.ID) && (x.Quantity < item.MaximumStackableQuantity));

                    // Calculate how many more can be added to this stack
                    int maximumQuantityYouCanAddToThisStack = (item.MaximumStackableQuantity - inventoryRecord.Quantity);

                    // Add to the stack (either the full quanity, or the amount that would make it reach the stack maximum)
                    int quantityToAddToStack = Math.Min(quantityToAdd, maximumQuantityYouCanAddToThisStack);

                    inventoryRecord.AddToQuantity(quantityToAddToStack);

                    // Decrease the quantityToAdd by the amount we added to the stack.
                    // If we added the total quantityToAdd to the stack, then this value will be 0, and we'll exit the 'while' loop.
                    quantityToAdd -= quantityToAddToStack;
                }
                else
                {
                    // We don't already have an existing inventoryRecord for this ObtainableItem object,
                    // so, add one to the list, if there is room.
                    if(InventoryRecords.Count < MAXIMUM_SPACES_IN_INVENTORY)
                    {
                        // Don't set the quantity value here.
                        // The 'while' loop will take us back to the code above, which will add to the quantity.
                        InventoryRecords.Add(new InventoryRecord(item, 0));
                    }
                    else
                    {
                        // Throw an exception, or somehow let the user know they are out of inventory space.
                        // This exception here is just a quick example. Do something better in your code.
                        throw new Exception("There is no more space in the inventory");
                    }
                }
            }
        }

        public class InventoryRecord
        {
            public ObtainableItem InventoryItem { get; private set; }
            public int Quantity { get; private set; }

            public InventoryRecord(ObtainableItem item, int quantity)
            {
                InventoryItem = item;
                Quantity = quantity;
            }

            public void AddToQuantity(int amountToAdd)
            {
                Quantity += amountToAdd;
            }
        }
    }
}

ObtainableItem.cs

using System;

namespace BusinessObjects.RedditRPGQuestion
{
    public abstract class ObtainableItem
    {
        public Guid ID { get; set; }
        public string Name { get; set; }
        public int MaximumStackableQuantity { get; set; }

        protected ObtainableItem()
        {
            MaximumStackableQuantity = 1;
        }
    }
}

Plant.cs

namespace BusinessObjects.RedditRPGQuestion
{
    public class Plant : ObtainableItem
    {
        public Plant()
        {
            MaximumStackableQuantity = 50;
        }
    }
}

Potion.cs

namespace BusinessObjects.RedditRPGQuestion
{
    public class Potion : ObtainableItem
    {
        public Potion()
        {
            MaximumStackableQuantity = 10;
        }
    }
}

Weapon.cs

namespace BusinessObjects.RedditRPGQuestion
{
    public class Weapon : ObtainableItem
    {
        // No constructor needed, since we want to use the base class default of MaximumStackableQuantity of 1
    }
}