As a single user you can legally install it on two but can only use one at a time. EULA allows you the equivalent of a work and a home install. (It has been known to install on more without problem)

Resolution for '2018 software issue: 'I just purchased the Quickbooks Desktop Pro 2018 software, how many computers can I install this on?' available: Yes.

The 2018 software 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 2018 software issue yourself by looking for a resolution described in the article. If it is a complex 2018 software issue or you are unable to solve the issue, you may contact us by clicking here or by using other 2018 software support options.

For more information about resolving issues related to 2018 software in Quickbooks/QBO or to get support for the issue 'I just purchased the Quickbooks Desktop Pro 2018 software, how many computers can I install this on?', click the link below: