I think I got 26:
Kingdom is: Squire, Stonemason, Swindler, Noble Brigand, Cache, Baker, x, x, x, x
You are player 3.
P1 (hand of Necro, CCCC) buys NB; P2 reveals OE and Hovel, gaining Copper; you reveal OE and Hovel, gaining Copper. P1 draws OE, Hovel, CCC
P2 (hand of Necro, CCCC) buys NB; P1 reveals CC; you reveal CC. P2 draws CCC, (re-shuffle), NB, C
You (hand of Necro, CCCC) buy Stonemason, Squire, Squire; draw Copper, (re-shuffle), Necropolis, Squire, Squire, Copper
P1 buys Swindler. P1 draws CC, Necro, NB, (re-shuffle), Swindler
P2 plays NB; P1 reveals CC; you reveal OE and Hovel, gaining Copper. P2 has $5 and buys X. P2 draws CCCC, Necro.
You play Necro, Squire for buys, Squire for buys, spend coin token, buy Cache, Copper, Copper, Copper, Copper; draw CCCCC
P1 plays Necro, NB; (P2's reveal doesn't matter); you reveal Stonemason, (re-shuffle), OE, gaining Copper; P1 plays Swindler, hitting your Cache, giving you Cache. P1 buys NB; (P2's reveal doesn't matter); you reveal Hovel, Necropolis, gaining Copper. (P1's draw doesn't matter.)
P2 buys NB; (P1's reveal doesn't matter); you reveal Squire, Squire, gaining Copper.
At the start of your turn, your deck consists of 19 Coppers, 1 Stonemason, 2 Squires, 1 Necro, 1 OE, 1 Hovel, and 1 Cache, for 26 total cards.
Someone should check that to make sure the shuffles line up correctly and stuff.
JTB may also be onto something. I would not be surprised if it's possible to do better.
Edit: Now that I've seen florrat's solution, mine can be improved by two cards if Beggars replace Squires.