limit="" parameter to VarWEBLIST to limit the number of webs.
Add also a overlimit="" parameter to show an alternate text if there are more webs than the limit specified.
%WEBLIST{
format=" * [[$name.%HOMETOPIC%][$indentedname]]"
limit="20"
overlimit="[[%SYSTEMWEB%.SiteMap][more...]]"
}%