Determining how many lamps can be connected

It depends on the wire gauge which in turn depends on factors such as current draw of each lamp, the max allowable voltage drop at the end of the wire

The following is an example calculator for pumps
http://www.paigewire.com/pumpWireCalc.aspx

You want to cut the plugs off of the light fixtures, splice the wires onto one cable, bury the cable and plug it into a 120 volt circuit via a timer?
Not Safe. Not Legal. No Way. No How.
120 volt floodlighting is far different than low voltage landscape lighting.
The cordage on the fixtures is not rated for burial.
Splices must be made in approved devices.
The landscape wiring is not approved for line voltage.

@randy427

+1

@OP

All that miscellaneous stuff he said boils down to one thing: death is waiting. For your dog. For you. For some neighborhood kid.