Currently, to implement this, I have been creating a different mechanic for each number I plan to use.

Mm, I've wanted this as well. It ought to be a nice simple edit to just one file (which is sadly the kind of criterion I'm working by at the moment: big architectural changes are beyond me with the level of energy I have right now). So I'll definitely have a go at this.

This is nearly ready. As well as the current confusing \1 and \2 in reminder text, you can get the same effect by writing PARAM1 and PARAM2. To convert a number to words, use PARAM1ASWORDS or PARAM2ASWORDS (or the old-style \1ASWORDS or \2ASWORDS).

For example, to implement Reinforce as on Hunting Triad, define the reminder text to be:

> PARAM2, Discard this card: Put PARAM1ASWORDS +1/+1 counters on target creature.

I haven't got a good solution for the version where you want "a +1/+1 counter" rather than "one +1/+1 counters" (Earthbrawn), though.

Edit: Hm, that was displaying weirdly. Fixed.

I would suggest that you can probably get away with just up and replace "one" with "a" (at least it would work for all existing official mechanic templates), but I'm willing to bet there are already multiple mechanics on this site with templates like "/1 mana of any color" or "up to /1 X", which would require "one".

Hm, good point. But I don't really mind "put one +1/+1 counter on it". The problem is the plural: "put one +1/+1 counters on it" is what bugs me. But to work around that I'd need to provide something akin to Inflector::pluralize, which seems somewhat incompatible with having a nice comprehensible approachable syntax.