It takes 215 alchemy/enchanting master writs to get to level 50, assuming your only xp buffs are eso+ and a 150% xp potion or scroll.

Glad someone provided a short answer! The long answer depends on a number of factors, including which type of writs you're doing.

UESP has a table of experience required at each level and the standard xp reward by level here: https://en.uesp.net/wiki/Online:Leveling

For master writs, the xp reward firstly depends on the type of writ you're doing. Consumables (enchanting, alchemy, provisioning) are considered "Hard," granting 1.5x the standard xp reward, while durables (blacksmithing, clothing, jewelrycrafting, woodworking) are considered "Very Hard" and provide 2.25x the standard xp reward.

Then you get 10% extra xp for having ESO+, and another 10% xp for wearing a Ring of Mara while in group and in range of your Mara partner (provided they are wearing their ring too). Then also add any xp buff you're getting from scrolls/pots (50%, 100%, 150%), and any event bonus. Those are all additive and based on the original values of the quest reward (the 1.5x or 2.25x figure, depending on writ type).

Most other experience bonuses do not apply because they only work on xp gained in combat (e.g. the 10% for just being grouped with one other person, the 15% from Mora's Whispers, any training trait with or without Heartland Conqueror).