The amount of TOTPs will only be limited by the available external flash memory, which is 2MB. This should be quite some space, so 500 should be easy, 5000 might be challenging already and depending on the other things that are kept within the external memory. Anyways the plan is to not restrict the amount of TOTPs, moreover the memory will be the limiting factor. Life expectancy is "long", e.g. Nitrokey 3 is a long term support product, similar to the Nitrokey Start/Pro, which we are now selling and developing for more than 5 years. There are no plans for a specific (OATH/TOTP) app right now, but we are looking into this.