How do you capture a wizard?

[Jack_Simth]

Where exactly do the RAW give the number of Solars?

It's not a house rule, but it is a pretty big logical assumption. It is all more or less theoretical, as infinites don't exist in reality, but I see no reason why an infinite space cannot contain a finite number of souls, or even an infinite number of souls containing a finite number of potential solars.

Solars are harder to prove (takes more sources), but Planetars (which mostly have the same effect) are a few simple connections in the DMG.

1) DMG page 167: Heavenly Encounters Table: Planetar is on the list. Additionally, Beatific Encounters Table: Planetar is on the list. Any time you're on a plane that uses either of those two encounter tables, there is a positive density of Planetars on that plane that is noticeably above zero (as they can be encountered at random).
2) There are a number of planes, defined as infinite in size, that use one or both of those encounter tables: Heroic Domains of Ysgard (page 158), The Clockwork Nirvana of Mechanus (page 163), and The Concordant Domains of the Outlands (page 166). All of these are strictly Core.
3) When you have a positive density of something that comes in discrete units in a volume that is defined as infinite, you must, of necessity, have an infinite number of that something. Thus, there are an infinite number of Planetars.

(A similar method can be used to prove that there are an Infinite number of Noble Dijinni and Efreeti - still staying in Core)

If you branch out of Core, the Planar Handbook adds Solars to the encounter tables of The Seven Heavens of Celestia, the Twin Paradises of Bytopia, the Blessed Fields of Elesium, the Olympian Glades of Arborea, and the Wilderness of the Beastlands. Meanwhile, the Manual of the Planes (Directly referenced in the Planar Handbook!) defines the The Seven Heavens of Celestia as infinite in size, the Twin Paradises of Bytopia as infinite in size, The Blessed Fields of Elesium as infinite in size, and the Olympian Glades of Arborea as infinite in size. The same logic in core-only that says there are an infinite amount of Planetars also says that there are an infinite amount of Solars, once you've reached outside of Core (Oh yes, and the Manual of the Planes puts Solars on the Heavenly Encounters table, as well as the Beatific Encounters table, as well).