max. number of IP free ip per subnet for multiple ALB

This actually depends a lot on how big the ELBs might scale. Each ELB could use up more than 100 IPs (Private & Public), so if they have 8 ELBs that all are taking the maximum amount of traffic they will need 800+ IPs.

When creating an ELB we check for 8 free IPs in each subnet to be used and won't let you create one without it. However, 8 is really the minimum we need for a single ELB to handle most workloads and failure cases safely.

Best practice is to give ELBs their own subnet(s), either by making one large (/18 - /21) one per AZ or by making a new one (like a /25 or /26) for each ELB with.