a % of the total. yes if you know how to do it, you can set it manually but usually is automatically 1 %
https://bitcoin.design/guide/how-it-works/liquidity/#channel-reserve

But there are more...

• anchor reserves
• commit_fees that vary if the mempool fees are spiking
• buffer channel reserves

so many fucking reserves that if you open a shity tiny channel you end up with almost nothing available to use.