QB Issue Resolution:

You can’t.  Once you hit the 14,500 mark it’s locked and you’re stuck.  You can restore a pre-lock backup and merge some old entities, but the only way to increase the limit is to upgrade to Enterprise.

Resolution for Issue 'How can we increase the maximum number of customers/vendors in QB Premier Windows Desktop above 14,500?' available: Yes (Solved).
Source: Intuit Community forum.

How can we increase the maximum number of customers/vendors in QB Premier Windows Desktop above 14,500?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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