I also like non-infinite but superexponential scaling. Dovescape + Mana Echoes + Spells that cost X.
Let's say you start with a 3 mana spell.
3 doves get created, each "sees" 2 others as they come into play, so 6 mana, 3 doves.
Use 6 mana to make 6 more doves. Each "sees" 8 doves, so 48 mana, 9 doves.
Use 48 mana to make 48 doves, each "sees" 56 doves, so 2688 mana, 57 doves.
Use 2688 mana to make 2688 doves, each "sees" 2744 doves, so 7375872 mana, 2745 doves.
etc, etc.
Where n is the step, d is the initial number of doves under your control, and m is the mana spent on the first X spell.
This scales much, much faster than a simple exponential function, but isn't infinite so long as you're careful not to have a way to endlessly re-cast the X spells.
Dovescape + Mana Echoes + Spells that cost X.
Let's say you start with a 3 mana spell.
3 doves get created, each "sees" 2 others as they come into play, so 6 mana, 3 doves.
Use 6 mana to make 6 more doves. Each "sees" 8 doves, so 48 mana, 9 doves.
Use 48 mana to make 48 doves, each "sees" 56 doves, so 2688 mana, 57 doves.
Use 2688 mana to make 2688 doves, each "sees" 2744 doves, so 7375872 mana, 2745 doves.
etc, etc.
$Doves_{1}=d$
$Mana_{1}=m$
$Doves_{n}=Mana_{n-1}+Doves_{n-1}$
$Mana_{n}=(Doves{}_{n-1})(Mana_{n-1}-1)$
Where n is the step, d is the initial number of doves under your control, and m is the mana spent on the first X spell.
This scales much, much faster than a simple exponential function, but isn't infinite so long as you're careful not to have a way to endlessly re-cast the X spells.