What is the default scheduler pool size in spring-boot?

Yes, all @Scheduled methods share a single thread by default.
It is possible to override this behavior by defining a @Configuration such as this:

@Configuration
public class SchedulingConfigurerConfiguration implements SchedulingConfigurer {

    @Override
    public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
        ThreadPoolTaskScheduler taskScheduler = new ThreadPoolTaskScheduler();
        taskScheduler.setPoolSize(100);
        taskScheduler.initialize();
        taskRegistrar.setTaskScheduler(taskScheduler);
    }
}

This example ensures that all @Scheduled methods share a thread pool of size 100.