In every modern economic system, there is a share of individuals in the labor force who are in between employment. What that share is depends entirely on the institutions of the systems those people are in, and so there isn’t one golden rule figure to spit out.

Absolutely. It’s like with housing, in an ideal system there will be some vacant housing but everyone will be housed. There is going to be some give to the system as people move around between houses.

Similarly, people moving between jobs might have a period of unemployment, and that’s okay. What is vital is to have a safety net that lets them more or less maintain their standard of living so they’re not dissuaded from reaching for better opportunities. A weak or non-existent safety net hands power to employers because employees may not have the option of leaving a bad job to seek out something better.

Natural? Absolutely not.

In a capitalist system, where labor is a commodity bought by employers, it will tend to have unemployment. Unemployment is extremely useful for capitalists primarily because it drives the prices of labor down, because unemployed people will be more desperate for a job and accept lower wages, and also it leaves a larger pool of labor to be employed anytime needed.

The market itself cannot fix unemployment, and in fact, tends to worsen it. Unemployment is only fixed through state intervention and programs to employ people. The lowest unemployment rates are found in countries which do not obey the capitalist market logic, namely socialist countries. Cuba had an unemployment rate of 1.2% before the pandemic, Laos has an unemployment rate of 1% and was lower before the pandemic, the USSR had rates of unemployment below 1%.

There’s nothing natural about unemployment. It’s a state policy to leave a high percentage of the people unemployed, and it only benefits corporate interests.

there is under capitalism

Yes. Around 99.998%, given current technology. Source: I am a technologist by profession.