The three ceilings
Structure decides the edition, and you can check it in an afternoon
Three ceilings decide the edition, and none of them is headcount. Can Salesforce hold the things your business talks about? Does another system need your data? Do two teams need the same object to behave differently?
One, the modelling ceiling. Starter allows zero custom objects, zero custom apps and 25 custom fields per object. That is the design, not a flaw: it suits a business already described by accounts, contacts, leads, opportunities and cases. Need a noun Salesforce does not ship with and Starter cannot hold it.
Two, the integration ceiling. API access is included by default in Enterprise, Unlimited, Developer and Performance. Professional can buy it, and the SKU is the trap: Salesforce states that Additional API Calls does not enable API access for Professional Edition, and that you must order the Web Services API product instead.
Three, the variation ceiling. Record types are not available in Starter, and active validation rules cap at 20 per object below Enterprise against 200 above it. So the third ceiling is variation: two teams needing one object to behave differently.
Allocations change, so treat this as the shape of the question rather than a price list. Salesforce publishes the current edition comparison, and your account team remains authoritative.