Recent enough to reflect today's market, and numerous enough to mean something — which are often in tension.
Three months is a sensible default in a normal market. Six months is usable with care. Beyond that you are describing a different market and the numbers need adjusting rather than accepting.
How much recency matters depends on how quickly conditions are moving. In a flat market a five-month-old sale is still informative. In a rapidly changing one, a sale from two months ago may already be misleading. Current conditions →
Three genuinely comparable sales is a reasonable minimum. One is an anecdote. The tension is that tightening recency and similarity at the same time can leave you with nothing — particularly for an unusual property or a thin segment.
When that happens, widen deliberately and say which dimension you widened: comparable homes slightly further away, or slightly older sales, or slightly different sizes. Widening on one axis and adjusting for it is defensible. Quietly widening on all three is how people talk themselves into a number.
Sold prices and property history require a free registered account under Alberta's rules for MLS® data — a licensing requirement rather than a paywall. Once you have one, you can check comparables yourself.