With on-demand and always-on fleets, it takes 10-15 minutes for another instance to become part of the fleet and available as a streaming resource. You should adjust your scaling polices so you have a larger amount of extra instances ready (buffer capacity).
In this case, you could change your default-scale-out to
If Capacity Utilization >= 50 % then add 2 instance(s), which would make your fleet larger after 50% of the fleet instances are used.
Scaling is an in-depth conversation. Here is a good blog https://aws.amazon.com/blogs/desktop-and-application-streaming/scale-your-amazon-appstream-2-0-fleets/. If you know your account team, you should reach out and a specialist can go through this in more detail.
With the settings you provided your fleet starts with only two available instances and those would be consumed by the first two URLs to be used. As your instances are used your scaling settings come into play where the back-end will spin up new instances for your users. There is a delay associated with the pending capacity but it should eventually make new instances available up to your maximum of 5.
If you want all four URLs to be usable immediately you need to adjust your minimum capacity to match. Please see here for more details: https://aws.amazon.com/blogs/desktop-and-application-streaming/scale-your-amazon-appstream-2-0-fleets/
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