I watched something on YouTube a while back about this, but they were using a standard pack of 52 playing cards. I wonder, does a 78-card tarot deck require more shuffles to randomize, being that it has 50% more cards? Of course, this is for riffle shuffling. For overhand shuffling, they said maybe more than 20 times would still not fully randomize the cards. But all of this is academic.
Agreed with others that you should have your method established before shuffling so the cards "know" where to be, or know what to expect when being shuffled. I riffle 8 or 9 times, then overhand about as many, and then cut into two piles. Decks that I can't/won't riffle just get overhanded a couple dozen times and cut. I like to cut by fanning across the table and picking up one side from wherever the cut card was chosen, as I feel cutting from a stacked deck (no pun intended) tends to cut at certain cards that don't lay perfectly flat with the rest of the deck.
For decks that I only overhand shuffle, I like to insert the cards from the reading back into the deck at random, instead of piling them on top to reshuffle. I think that keeps the deck a little more randomized, given that overhand shuffling tends to keep cards in clumps.