When should I start Hearts of Stone?

Recommended level is 30 or higher. The standalone start gives a preset level 32.

When should I start Blood and Wine?

Recommended level is around 34, with a standalone preset level of 35.

How many endings are there?

Wikipedia says there are 36 possible endings that affect characters' fates, with three main outcomes for Ciri.

Witcher 3 DLCs and Endings Explained (Spoiler Labeled)

The Witcher 3 has two big expansions and an ending that depends on your choices. Here is when to start each DLC and what the main outcomes are. Spoilers appear in the last section.

Key takeaways

Hearts of Stone: released October 13, 2015, recommended level 30 or higher.
Blood and Wine: released May 31, 2016, recommended level around 34.
Both are included free in the Remastered edition.
Wikipedia counts 36 possible endings, with three main outcomes for Ciri.

The Witcher 3: Wild Hunt – Blood and Wine – Launch Trailer · YouTube

Hearts of Stone

Geralt takes a contract from Olgierd von Everec and meets Gaunter O'Dimm, adapted from the folktale of Pan Twardowski. It adds the Runewright, who upgrades gear with runes. IGN and GameSpot gave it 9 out of 10. Recommended level is 30 or higher, and the standalone start gives a preset level 32.

Blood and Wine

Set in the duchy of Toussaint, Geralt hunts a beast that is killing knights. It won Best RPG at The Game Awards 2016 and has a Metacritic score of 92 on PC. Recommended level is around 34, and the standalone preset is 35. According to Gfinity, you start it with the quest "Envoys, Wineboys" after "A Poet Under Pressure", from the notice board in Mulbrydale in Velen. The reward is a vineyard estate.

Endings overview (spoilers)

Wikipedia lists three main outcomes for Ciri: she becomes empress of Nilfgaard; she fakes her death and becomes a witcher; or she dies, and Geralt then hunts the last Crone. In total, 36 possible endings affect the fates of other characters.