These are some new modes that should be released in a future update. Here's one of them:
Max 10
Where it is in mode selection screen if it were an actual mode: Next to Deflation Mode
Description: Survive through wave 65 on easy, but you may have no more than 10 towers on the field at one time!
(Special Agents, Road Spikes, & Pineapples are disabled.)
And here's another one:
Max 5 (You unlock it for any track by beating Max 10 on the track in question.)
Where it is in mode selection screen if it were an actual mode: Next to Sandbox Mode Description: Survive through wave 65 on easy, but you may have no more than 5 towers on the field at one time!
(As like before, Special Agents, Road Spikes, & Pineapples are disabled.)