When purchasing for Pearson VUE AWS Exam voucher for any one of the pack, can we use it to write all the exams within it ?

Based on the available information, AWS certification exam vouchers are typically designed to be used for a single exam registration, not multiple exams. Each voucher code can only be applied to one exam at a time, even if multiple exams are listed within a certification category or "pack" as shown in your image.

When you purchase a voucher for a specific certification level (like Foundational, Associate, Professional, or Specialty), that voucher can be used for one exam within that category. For example, if you purchase a Foundational level voucher for $100, you would need to choose either the Cloud Practitioner or the AI Practitioner exam, not both.

The pricing structure shown in the image appears to be indicating the cost for a single exam voucher within each certification level category, not a bundle that includes multiple exams for one price.

After passing one AWS certification exam, you automatically receive a 50% discount voucher that can be applied to your next certification exam. This is one way AWS provides discounts for taking multiple exams, but it still requires separate vouchers for each exam.

If you're looking to take multiple AWS certification exams, you would need to purchase separate vouchers or use discount codes for each individual exam registration.
Sources
Stacking Vouchers for AWS Certification Exams | AWS re:Post
retaking exams certif | AWS re:Post
Cloud practitioner exam voucher | AWS re:Post

Vouchers are typically used by employers/organizations that want to support their teams to get AWS Certified (see https://aws.amazon.com/certification/bulk-voucher/ for more information). Individuals looking to register for an exam are not required to purchase/use a voucher as you can submit credit card payment through the appointment scheduling flow.

If you purchase a Foundational voucher, it will be available for single use on a Foundational exam. So you could use it to schedule an AWS Certified Cloud Practitioner exam, but you could not then go and use the same voucher to schedule an AWS Certified AI Practitioner exam (unless you had canceled your AWS Certified Cloud Practitioner exam appointment).

As an individual, there is no benefit to purchasing a voucher and then using that to "pay" for your exam.