16g cartridges quickly inflates 700c tyre and will fill most 26-27.5" tyres. 25g cartridges are great for larger and wider tyres such as 29" tyres and plus sized/fat bike rubber. You're better off having more on hand than you need rather than running out before the tyre gets to a rideable pressure.
Always a good idea to keep a few extra on hand! Requires a CO2 inflator.