Jetblack C02 cannister 16gm
16g cartridges will fill most 26" and narrower 27.5" tyres (one cartridge, one tyre). 25g cartridges are more suited to wide 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.
Be sure to keep a few extra on hand!