The no. of simultaneous searches you can have is really only limited by your bandwidth. And the no. of simultaneous searches can be set here: Windows: Tools>Options>Searching>Limit.
Mac OS 9: Options>Searching>Limit or Mac OSX: LW's Prefs>Searching>Limit
As far as previous searches go, if you have a certain no. they will be hidden. But to the right of the search name of the search listed furtherest to the right, there will be an arrow which you can click & a small drop-down window will show that lists all your previous searches. Of course you can delete previous searches by clicking on the red cross to the left of the search name. Keep in mind that after some time search results become stale. People constantly enter/leave the network & also upld slots become available or not from time to time.