[logback-user] Spring and Logback : 8 threads (logback-1 to logback-8) are created.. why 8 ?

Dear all, *first of all, excuse me if you receive this issue 2 times, but I've sent it before subscribing and I was not sure it was accepted.* I have deployed 10 applications in Tomcat/Spring and each one contains logback in order to have a different log file for each application. When I look at tomcat threads, I see 8 threads per application (named logback-1 to logback-8). Does anyone know why 8 and how to reduce that number ? (and the consequencies) ? 8x10 is much ... thanks in advance for any help, Samuel