I have aluminum 12 2 wiring i want to add an extra light Daisy chain what gauge/amp copper wire do i use for a 3 ft span, i will be using alumiconns
You'll want 14/2 NM...

The rule for converting between aluminum and copper wire gauges is that in order to carry a given amount of current, you need an aluminum wire two sizes larger than the copper wire you would have used.  So, a 12AWG aluminum wire is rated for the same 15A as a 14AWG copper wire is, and NM is apropos since jurisdictions that don't permit NM also generally don't permit the use of aluminum, especially at branch-circuit gauges.

...and a torque screwdriver

While your choice of AlumiConns for this job is a good one, as they are the most reliable way of making an aluminum-to-copper branch circuit wiring connection available to the rest of us, they, like all mechanical setscrew splice/tap connectors, are sensitive to tightening torque.  As a result, a using an inch-pound torque screwdriver to tighten the connector screws to the manufacturer's specified torque value is highly recommended for long-term reliable results.

(Sidenote: if you're having trouble with the relatively chunky Alumiconns taking up too much room in the box, get a Wiremold ceiling starter box and use it as an extension ring to add extra volume.)