Subscriptions until June 2021 were based on document number. Because the subscriptions are recurring, when you bought the old one, you still have a subscription with previous features.
All new subscriptions since June 2021 have unlimited documents but limited client number.
An overview of old subscriptions:
Basic
document types: invoices and expenses
number of documents: 120 annually or 10 monthly
1 team member
unlimited clients
Starter
document types: invoices and expenses
number of documents: 30 annually or 3 monthly
1 team member
unlimited clients
Standard
document types: all
number of documents: 300/360 annually or 25 monthly
1 team member
unlimited clients