The Web container uses this number to determine the order in which to load servlets. If a <load-on-startup> tag is not specified, the servlet will not be loaded on startup. If only one servlet has a <load-on-startup> it doesn't matter what number appears: it will simply be loaded. If more than one servlet has a <load-on-startup>, the web container will load them in order from the lowest number to the highest.
So, in answer to your question: If servlet A has a value of 1 in <load-on-startup> and servlet B has a value of 100, all it means is that servlet B will be loaded after servlet A.