How many Shopify apps is too many?
More than about eight, and the real limit is whatever slows the storefront. Each app adds scripts to every page, and the cost is paid by every visitor on every visit.
In more detail
Audit them quarterly against what they changed. The usual finding is three or four installed during a problem that was solved a year ago, still charging monthly and still loading on the product page.
What to do about it
Most Shopify performance problems are the theme and the images, not Shopify.
Shopify handles the parts that are genuinely hard and genuinely regulated: payments, taxes, order records. That is what the subscription is for.
The apps in the Shopify store are where the cost quietly grows. Six at twenty dollars a month is more than the plan.
The mobile app angle
A mobile app sits on top of Shopify rather than replacing it — the catalogue syncs, and checkout stays Shopify’s.