I have added 7 inbound rules, 5 of them are with "Managed prefix list" that has 2 entries, those entries are public IPs with /32 CIDR block, so I am not able to add more inbound rules with "pl" as the max count gets reached, but how the figure "52" is calculated ?
>Solution :
When you reference a prefix list in a resource, the maximum number of entries for the prefix lists counts against the quota for the number of entries for the resource. For example, if you create a prefix list with 20 maximum entries and you reference that prefix list in a security group rule, this counts as 20 security group rules.
— docs, emphasis mine
tl;dr: You define a max length for each prefix list that’s taken into account when the number of (inbound) rules is calculated.