Adding a new tenant VS adding Subscription

we have a tenant in our Azure company name XYZ with some App services and B2C users, now the company has acquired another company as a subsidiary.

Cool!

we want to enable Azure and Host some apps, Looking for suggestion in this case should I choose to create a new tenant or add as new subscription and manage it

But you just said you already have a tenant, do you mean you want to create another tenant for the company you guys acquired? but why? just add the new users to your already existing tenant? do that unless the new company has an already existing tenant, then use the Azure B2B features and add the users as "Guests" in your own tenant: https://docs.microsoft.com/en-us/azure/active-directory/external-identities/what-is-b2b

I would say read the best practices Microsoft has for:

• Identity Management: https://docs.microsoft.com/en-us/azure/security/fundamentals/identity-management-best-practices
• Subscription Management: https://docs.microsoft.com/en-us/azure/cloud-adoption-framework/decision-guides/subscriptions/

Based on your requirements and company size, adopt any of the recommendations on those docs.