Your reseller account has an overall allocation, and the accounts inside it draw from it collectively.
WHAT TO WATCH
WHM > Account Information > List Accounts shows disk and bandwidth per account, and your own totals. Client Area > Services > My Services shows your package limits. Individual accounts show their own resource usage in their cPanel under Metrics > Resource Usage.
THE FIGURES THAT MATTER
Total disk against your allocation Total inodes, which many resellers hit before disk space, particularly with several WordPress sites and mailboxes Bandwidth against your monthly allowance CPU and memory pressure, which on a shared reseller plan is affected by every account you host
SIGNS OF TROUBLE
One account consuming disproportionate CPU, usually a site with no caching or one under attack Inode count climbing steadily, usually backups and cache files accumulating Bandwidth spiking, usually bot traffic or a site serving large media
WHAT TO DO
Identify the account responsible before assuming you need a bigger plan. Enabling caching on one busy client site frequently resolves what looks like a capacity problem.
Open a ticket if you want us to look at the graphs with you.