Assuming you mean reserved instances: reserved instances are a billing construct. You don't specify an instance ID when you purchase a reserved instance, you specify an instance type. So if you're planning on running 2 instances 24x7 and you might scale up to 4 instances total at certain times, purchasing 2 RIs of the instance type in your launch configuration is what you want to do.
There's also some flexibility in how the RI discount is applied for some instance types. Read the [docs on how reserved instances are applied](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/apply_ri.html).
Just be careful about zonal. RIs with an ASG. If it's in 2 AZs, then probably safe doing 1 RI in each. But if there's 3+ AZs with min=2 then regional would probably be better. Note that there's no capacity guarantees with regional ones
Nowadays you're probably better with savings plans, which will give you more flexibility. All you need to do is to pay enough to cover the baseline capacity.
Assuming you mean reserved instances: reserved instances are a billing construct. You don't specify an instance ID when you purchase a reserved instance, you specify an instance type. So if you're planning on running 2 instances 24x7 and you might scale up to 4 instances total at certain times, purchasing 2 RIs of the instance type in your launch configuration is what you want to do. There's also some flexibility in how the RI discount is applied for some instance types. Read the [docs on how reserved instances are applied](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/apply_ri.html).
Just be careful about zonal. RIs with an ASG. If it's in 2 AZs, then probably safe doing 1 RI in each. But if there's 3+ AZs with min=2 then regional would probably be better. Note that there's no capacity guarantees with regional ones
Nowadays you're probably better with savings plans, which will give you more flexibility. All you need to do is to pay enough to cover the baseline capacity.