With even a handful of miners it is essential to carry spares. It is not worth returning a miner for faulty fan or PSU, because they are so cheap and you'll lose more in downtime, plus the risk of damage in the post.

I have approx. 200 miners total across 5 small farms. I reckon I get through at least 50 fans and 5 PSU's each year, and 1 or 2 control boards.

I'd recommend to keep as spares:

1 PSU for each 20 miners (with min 1 in stock at all times)
1 control board per 50 miners (with min 1 in stock at all times)
1 fan per 2 miners

I buy all my spares from ZeusBTC (import from China).