ProofHub provides online project management solution that comes with a clean, simple interface and unlimited users. It's priced $15-$149 a month, depending on how many projects you need to manage, as well as which features need to be available. There is no information about self-hosted edition of ProofHub, so most likely there is no way to deploy it on-premise or get access to source code. If you are looking for a free ProofHub alternative that doesn't have these limitations, take a closer look at Bitrix24.
* Please note that the information may have changed since the publication. For current prices and features please visit Bitrix24 pricing page.