It is dangerous to bike with 5/36 spokes broken?
Definitely not recommended to ride a wheel with broken spokes. Even if the wheel does not catastrophically break and result in injury, the bike could leave you stranded.

If the bike is old and has not been maintained, you should get it inspected for other safety issues, especially proper functioning of the brakes.

There are a few questions on here about inspecting a bike for safety issues. Here's one I found after a quick search: Safety check for a used bike