Theoretically, what would be the highest cloud that could be generated by an updraft?

The highest a cloud can get due to air rising in the troposphere is the tropopause, the boundary between the troposphere and the stratosphere. Temperatures in the stratosphere increase with height preventing air from rising so cloud spreads out at the tropopause. We see this most commonly in the anvil of cumulonimbus cloud where the rising air spreads under the tropopause.

That being said, in some severe thunderstorms, the strength of the updraft is such that it punches into the stratosphere forming a dome of cloud above the anvil. That would be the highest a cloud could form in an updraft.

The tropopause varies in height from 60,000ft+ in equatorial regions to, perhaps, 12000ft at the poles. The highest I have ever seen a such a cloud was a cumulonimbus over the Tiwi islands north of Darwin in Australia with tops in excess of 80,000ft - but that height would be rare.

The highest cloud-type produced by a convective updraft would come from a cumulonimbus (Cb)

A cirrostratus 'anvil' cloud will usually form at the top of a Cb; however...when the updraft is especially strong...it penetrates into the stratosphere as an over-shooting top.

The highest cloud that could be generated by an updraft is a Cb's over-shooting top.

Tento: You must be thinking Meters,

Ony cirrus clouds form above 20,000 feet, there is no Air at 80,000.

I have seen cumulonimbus at 10,000 in an airplane. No higher.

Cirrus aren't formed by updraft but by deposition. They are ice crystals, my daddy called 'em horse tails.