My bike has 21 gears the new wheel only has 18 gears w ill the bike work

Number of Speeds is easy. Count the number of cogs on the chainwheel (that's the one with the pedals), count the cogs on the rear wheel and multiply the two together. Normally 3 on the front and 6 one the back, so 18 gears, or 3 and 5 is 15, 2 and 5 is 10.

To remove the front wheel, turn the bike over, wheels in the air slacken off the wheel nuts on either side and slide the wheel up. You make have to slacken the brake to slide the wheel and tire out.

There are some manuals here. Ironhorse seems to be a budget brand available from places like Walmart and Canadian Tire

The search Query would be simply

make model manual

https://www.google.com/search?q=iron+horse+mountain+bike+manual&safe=off&rlz=1C1QJDB_enCA643CA644&sxsrf=ALeKk00PfZADYZomZle3qjXqE1cWk29ioA%3A1616456368052&ei=sCpZYJvGAoKw5NoPoJuXqA0&oq=ironhorse+bike+manual

Basic Mountain Bike Maintenance here

https://www.google.com/search?q=basic+mountain+bike+maintenance

Good luck and please remember to wear all the safety gear!
..

Hey, Local bike shop determined that both wheels were defective. I took the junk back to Walmart and got a refund... PROBLEM SOLVED!!

Check out https://gearly.org/ for more info.

Thanks.

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Bicycles usually have 1, 3, 18, 21, 24 or 27 gears. There used to be bikes with 10 and 15 gears but this are old and bike manufacturers do not manufacture them anymore.

you will find it on e-bay or from the manufacturer spares

It is the derailer (the part on the back that the chain runs through). Take it to a bike shop or bike enthusiast. The repair should only take about 5 min. Some bike shops won't even charge you.

If a single speed bike loosen the rear wheel and pull it back away from the pedals to get the chain tighter

If it has multi gears the arm hanging down has a ser screw that will need to be adjusted to align the chain to stop at the last gear

there are 7 cogs on the rear wheel hub; three on the crank hub:
7x3=21 gear change possibilities
large gears move the wheel faster and farther around but, with more effort
small gears move the wheel less distance but, with less effort
i.e. 10 teeth divided by 30 teeth= 3:1 per 360 degree rotation of the bicycle wheel

Turn the bike over so its sitting on the seat & bars..
There are a few small setting srews on the derailleur..
These are for adjuting.. Adjust while turning the peddles & shifting.. You should be able to smooth it out

If the gears will no longer go back on the bike, I would recommend buying a new wheel (including a new rear hub). It is usually easier and more cost effective to buy a wheel than to replace the hub. To replace the hub (the part that is stripped on your bike) you would also have to remove and replace spokes, etc. Search online for the wheel size you need (probably 26" - you can check the tire size) or take to a reliable bicycle shop and they will gladly help you. Anyway, you need a new hub but probably better off getting a new wheel - much easier.