To start with I had three catalogues. Weddings (a big one), Portraits and Commercial, and Personal/Family. I reasoned that there was no real cross-over in the three and that I would only need one open at a time. The problem came when say I wanted to say print a lot of pictures from the wedding catalogue, but get on editing a portrait shoot. So after two years I have just merged (painlessly) the Wedding and Portrait catalogues. The new catalogue has over 90,000 pictures in and runs well.
My own method is to download new pictures to specific locations on hard drives, so that I know where the little so and so's are kept for back-up purposes. Then I import them into Lightroom without moving them. The Lightroom catalogue is regularly backed up, and Time Machine (Mac) backs up the back ups! The image files are backed up separately.
I am tempted to combine the Personal catalogue with the other as well. The problem is that when doing viewings with clients it is very easy to flick the cursor across other sets of pictures, which then pop up in the viewer briefly. Whilst there is nothing dodgy in my catalogue, it is just not good to have ones personal family pictures popping up unexpectedly! Granny in the bath..... (only joking).
Jim