You can put as many as will fit in the body, plus a piezo bridge, plus build an arch over the top and put another layer.
you cuold probably cram about ten or more pickups in the bass. But you won't get a lot of control over it. Will each one get a volume control of its very own?
For a while I owned a bass with three J pickups, and a volume for each, and it was great.I probably wouldn't go more than 3 myself, but if you want to be extreme check out Traben's Bootsy Star bass with 5 J pickups.
Do you want to keep this bass for a long time? If so, try not to make it too outlandish, and try to include the classic pickup locations in your design somewhere and make sure it will be easy to get a classic selection (say the 70's fender jazz positions) with the flick of a single switch or knob. Don't make your bass so complicated that you can't use all the best bits without a minute's fiddling.