There is no limit as long as your database has enough hardware resources to handle the large search.

If your goal is to find all documents under a folder, you could also crawl the folder tree. This bypasses the search engine and avoids a large temporary table with the search results. Here is some sample code:

List<DocumentInfo> FindAllDocumentsUnderFolder(Session session, string path)
{
    List<DocumentInfo> allDocuments = new List<DocumentInfo>();

    EntryListingSettings settings = new EntryListingSettings();
    settings.AddColumn(SystemColumn.Id);
    settings.AddColumn(SystemColumn.Name);
    settings.AddColumn(SystemColumn.EntryType);
    settings.SetSortColumn(SystemColumn.Name, SortDirection.Ascending);

    Stack<string> folders = new Stack<string>();
    folders.Push(path);

    while (folders.Count > 0)
    {
        string currentFolderPath = folders.Pop();

        Console.WriteLine($"Processing {currentFolderPath}");

        using (EntryInfo currentEntry = Entry.GetEntryInfo(currentFolderPath, session))
        using (FolderListing listing = ((FolderInfo)currentEntry).OpenFolderListing(settings))
        {
            foreach (EntryListingRow row in listing)
            {
                int entryId = (int)row[SystemColumn.Id];
                string entryName = (string)row[SystemColumn.Name];
                EntryType entryType = (EntryType)row[SystemColumn.EntryType];

                string entryPath = EntryPath.Make(currentFolderPath, entryName);

                if (entryType == EntryType.Folder || entryType == EntryType.RecordSeries)
                {
                    // Add folders to the stack so they can be processed later
                    folders.Push(entryPath);
                }
                else
                {
                    allDocuments.Add(Document.GetDocumentInfo(entryId, session));
                }
            }
        }
    }

    return allDocuments;
}