Can you accumulate multiple respecs in Diablo II?

You can have one respec per difficulty by completing the Den of Evil quest from Akara in Act 1. These can be used at any time by starting a game in the appropriate difficulty: one in Normal, one in Nightmare and one in Hell.

Additionally, you can also create a Token of Absolution by transmuting the following items in your Horadric Cube:

1. Twisted Essence of Suffering from either Hell Andariel or Hell Duriel.
2. Charged Essence of Hate from Hell Mephisto
3. Burning Essence of Terror from Hell Diablo
4. Festering Essence of Destruction from Hell Baal

Note that these are all not 100% drops, and are in fact rather rare (< 10%), so it may take quite a few runs to get all the items you need for a full transmute.

The Token of Absolution allows a respec exactly the same as through Akara. Simply right click the token to consume it and respec.

Also note, this is only available in Patch 1.13. This is standard now on closed Battle.net play, but if you're playing single player or Open Battle.net and have not updated to 1.13, these respec methods are not available.

You can save them up, the same as the imbue, personalize or socket quest rewards.

To use them later you create a new (eg) normal game and use the waypoint system to get to the appropriate NPC to use the particular quest reward. You can do this at any time.

Also, you can make respec tokens so you'll never run out of respecs. It does take a little work to do though, as they drop fairly infrequently.