D&D 5e Variant Level Progression by # of Encounters
Please check out the attachments.
I made a quick spreadsheet to set up a variant PC level progressions for 5e. I've set the table in the attachments to take 12 encounters at average difficulty to progress from one level to the next. In other words, with this variant, a party of four 2nd-level PCs will need to kill 12 CR-2 monsters to reach 3rd level. You can set the table up to have non-linear, faster, or slower progressions.
A couple of notes:
1. To allow for "zero-level" play, I have set it so that it takes 200 XP to reach 1st level. If you prefer not to have zero-level adventures for your characters, just assume all 1st-level PCs start with 200 XP, or adjust all the XP requirements down by 200 XP.
2. I've included level progressions through Level 30. It always seems like D&D eventually includes epic-level play, so I've just extended the same logic using CR 21 through CR 30 monster-killing XP rewards.
Changing the Progression
I've included both a PDF and the Excel file so that you can set up your own progression tables.
Linear Progression
If you change the "Apx # Enc." (Approximate number of encounters) in cell F4 from 12 to some other number, the entire table will recalculate. This will allow faster or slower (but still linear) level progressions. Note that if you change this to 9.5, you will get very similar progression to the core rules, but the progression will be linear (i.e. you'll reach level 20 at about 355,000 XP, just like in the core rules, but you'll need only 44,000 XP to reach level 10, instead of 64,000 XP as in the core rules).
Non-Linear Progression
If you want your PCs to spend more (or less) time at certain levels, you can change the "Apx # Enc." for each level (Change all the values in Column F). For example, if you think Levels 5 to 10 are the sweet spot, and want to spend most of your time playing at mid-level, just change the Apx # Enc. in cells F8 to F13 to a higher number (you'll also need to hard-code the other values in column F). For reference, the Apx. # Enc. for the level progression of the core rules is shown.
Hope this helps someone out there.