A tube is needed if you get a puncture / gash that sealant won't fix, or if your sealant is all gone from sudden loss, or not keeping it topped up enough. Sealant level does need to periodically be checked, and how often is based on mileage / temps (more miles, higher temps = more often), or if you know you lost a lot from a puncture (usually there will be dried sealant all over your frame). Personally I just crack the tires open, drain what's in there out, reuse the old sealant if there was still a lot left (Orange Endurance), and refill to 2oz.
What's in my tool bag:
-Tubolito CX tube (expensive, but light and compact)
-co2 head with one 20g cartridge for regular rides, and an extra for weekend excursions
-Lezyne pocket drive pump with gorilla tape wrapped on it
-Basic tire plug tool with bacon strips
-Two small boots cut from old tubes
-Pre-glued tube patches (they weigh almost nothing and take up no space)
-Small cone shaped piece of plastic that is wedged in the strap of my saddlebag for very quick access to temporarily plug a hole (you need to plug big holes fast to minimize air / sealant loss)
I recommend practicing with your plug tool on a used tire, and also testing if your co2 and / or mini pump will inflate a completely flat tire that may have had the bead start to come off the rim. It's obviously better to get the tubeless fixed on a ride than having to go to the tube, but again, you want the tube as a backup too.