4x is pretty common in the publishing world and something in the 4x-6x range is usually a good choice for 35mm

10x or more is handy for checking sharpness, if you want to decide whether to make a big enlargement.

Whatever you decide, don't skimp on a loupe. Those cheap plastic 8x Agfa-type loupes will tell you less about your slides and will cause more eye strain than a high-quality 4x loupe. I use a Schneider 4x. If you don't have the budget for a Schneider or Rodenstock, the Peak loupes and the Pentax 5.5X have good reputations.