How many IPv4 addresses does ALB use by default?

Hello.

The number of public IPv4s changes depending on the number of subnets in which the ALB is placed.
In other words, if ALB is linked to 6 public subnets, 6 public IPv4s will be set.

If I configure the ALB to use only two availability zones, will it reduce the number of IPv4 addresses in my VPC?

So, the answer to this question is that if you reduce the number of linked subnets, the number of ALB public IPv4 addresses will decrease.
Looking at the Network Interface screen, you can see that the number of subnets and the number of public IP addresses match from the ALB ENI.

In my environment, when there are targets in three Availability Zones by linking ALBs to three subnets, you can see that the ALB ENI is created as shown below.

By the way, if the Availability Zone used by the ALB target does not match the Availability Zone of the subnet associated with the ALB, the health check will fail, so be careful when reducing the number of subnets.
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/target-group-health-checks.html#target-health-reason-codes