The maximum number of Sporebucks you can carry is 100 million minus 1 (99,999,999) Sporebucks.

Hitting the limit, while it doesn’t give you any achievements or badges (beyond the Golden Touch 5 badge for 10,000,000), does allow you to use the expensive ammo-based terraforming and combat tools, which can make life a little easier.

The way to get a constant stream of income is to “cultivate” homeworlds –

• Find a rare spice (pink, purple, green) planet.
• Terraform it to T3.
• Drop a monolith.
• Wait for the civilisation to become space faring.
• Set up a trade route.
• Wait for it to mature.
• Buy it over.

You can speed up the trade route step with the Trader Cash Infusion or Zealot Fanaticial Frenzy Superweapons. Or you can simply attack and save/reload until you take over the planet with all its colonies intact.

In most cases, it will cost 10,000,000 to buy over a homeworld, but it is possible to buy one for 5,000,000.

A planet cultivated in this way may have from one to ten colonies, depending on the amount of land the planet has. You might want to terraform the planet to have a larger land area.

Since each colony can store 10 spice (with a Spice Storage), it is optimal to have as many colonies per planet as possible.

You don’t have to find “clean” planets, as you can wipe out all the colonies (even your own) on a planet and drop a monolith to start over.

You can buy over a system from a friendly civilisation, then destroy the colonies which now belong to you. Similarly, if a civilisation forms with less than a satisfactory number of colonies, you can wipe them out and plant another monolith.

It is possible to have your entire empire comprised of former homeworlds, all with 10 colonies. A small empire, but a rich one.