For 900 leds, what power supply should I use? How many amps should it be? If it helps, it needs to be 5v
6amps per 100 is the general rule.
While that is the theoretical max, we have seen it demonstrated that it’s more like 3.3A 100 and 2A for dynamic effects (what wled suggests in the UI).
Always go for a psu that is well overrated especially if you going for a Chinese variant. I base my calculations on 45ma per pixel and then purchase a psu of at least a third greater on capacity. That way should should never run into any issues even at full white brightness.