All supports will defend any and all ally bases that come under attack, so long as those ally bases are within range of support weapon type. If a dozen bases are within range with each other and all 12 come under attack, then all 12 bases would eventually have 12 supports each (typically 8-10 min). The only thing presetting a support does is eliminate the need for that support to help that base. Typically, this is done when a particular base is holding a key position.