Freshness argues against a large collection
Green tea and light oolong are best within a year and noticeably duller after two. Black tea keeps two to three. Only aged white and dark tea have no useful upper limit. A shelf of twenty teas mostly means a shelf of teas past their best, and the strongest argument for buying small quantities more often is simply that you will drink them while they are good.
A working set for most people
Something for the morning that takes milk. Something fresh and green. One oolong. One dark tea for the evening or after a meal. That is four, covers most occasions, and can be rotated as things run out. Add a fifth slot for whatever you are currently exploring, and keep that one deliberately temporary.
Storing what you have
Opaque, airtight, cool, dry, and away from anything aromatic. A clear jar on a sunny worktop is the worst common option and the most attractive one. The exception matters: sheng pu-erh, pressed white tea and traditionally roasted oolong are kept to change, and sealing them airtight stalls exactly what they are being kept for. Those want a stable ventilated space instead.
When collecting becomes something else
Buying pu-erh to age is a different activity from buying tea to drink, with a long horizon, real storage requirements and a market whose age claims a buyer cannot verify. Both are legitimate; conflating them is how most people’s first expensive mistake happens. If you are buying something you do not intend to drink for a decade, know that that is what you are doing.