Form submit resulting in "InvalidDataException: Form value count limit 1024 exceeded."

The default formvalue(not formkey) limit is 1024.

Also, I think you can just change the FormOptions limit in Startup.cs file.

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<FormOptions>(options =>
    {
        options.ValueCountLimit = int.MaxValue;
    });
}

Update: The MVC SDK now includes this functionality via RequestSizeLimitAttribute. There is no longer any need to create a custom attribute.

Thanks to andrey-bobrov for pointing this out in a comment. The original answer is below, for posterity.

---

You can change the default formvalue limit using the FormOptions. If you are using MVC, then you can create a filter and decorate on action where you want to extend this limit and keep the default for rest of the actions.

/// <summary>
/// Filter to set size limits for request form data
/// </summary>
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
public class RequestFormSizeLimitAttribute : Attribute, IAuthorizationFilter, IOrderedFilter
{
    private readonly FormOptions _formOptions;

    public RequestFormSizeLimitAttribute(int valueCountLimit)
    {
        _formOptions = new FormOptions()
        {
            ValueCountLimit = valueCountLimit
        };
    }

    public int Order { get; set; }

    public void OnAuthorization(AuthorizationFilterContext context)
    {
        var features = context.HttpContext.Features;
        var formFeature = features.Get<IFormFeature>();

        if (formFeature == null || formFeature.Form == null)
        {
            // Request form has not been read yet, so set the limits
            features.Set<IFormFeature>(new FormFeature(context.HttpContext.Request, _formOptions));
        }
    }
}

Action:

[HttpPost]
[RequestFormSizeLimit(valueCountLimit: 2000)]
public IActionResult ActionSpecificLimits(YourModel model)

NOTE: If your action needs to support Antiforgery validation too, then you would need to order the filters. Example:

// Set the request form size limits *before* the antiforgery token validation filter is executed so that the
// limits are honored when the antiforgery validation filter tries to read the form. These form size limits
// only apply to this action.
[HttpPost]
[RequestFormSizeLimit(valueCountLimit: 2000, Order = 1)]
[ValidateAntiForgeryToken(Order = 2)]
public IActionResult ActionSpecificLimits(YourModel model)

In my case, it worked by changing ValueLengthLimit, in Startup.cs file

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<FormOptions>(options =>
    {
        options.ValueCountLimit = 200; // 200 items max
        options.ValueLengthLimit = 1024 * 1024 * 100; // 100MB max len form data
    });