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In the free tier, all titles you have are in Development Mode. The only limit Development Mode titles have that is different from Live titles is that they can only have 100,000 player accounts.

In general, you shouldn't be reading or writing more than 10 keys at a time. While it's technically possible to exceed that limit on reads if you're using multiple write calls to write to more than 10 keys: a) Bear in mind that the more data you store, write, and read, the more that spins the meters, so if you do reach the point where you go live, that will be generating costs. b) Exceeding that level by a lot (think 5x or more) starts to get into an area where it's feeling borderline abusive, so we may be reaching out to ask you to merge keys - and much higher than that would be getting into an area where realistically, you're outside the bounds of our fair use policy. c) I cannot recommend reading more than a few 100K into legacy Cloud Script. If you need large amounts of data, you should be using Azure Functions Cloud Script for this (which I'd recommend using in any case, as it has a number of advantages).