Hello! Sorry about the slow reply - only just seen this
Do you know what stick insects they are? I'm going on a hunch that they may be Indian Stick Insects as they are the most common stick insects to keep, I believe.
The tank should be large enough for Indian Sticks - this gives them enough height to shed safely (the general rule is the housing should be at least three times the height of the stick insect)
Numbers - seeing as the tank is quite roomy, you could get a few sticks to happily live in there. Indian Sticks can be 10-12cm when adult. I'm not entirely sure how many you should get - five or six should have enough room when they get to their adult size, but that is probably the maximum I would get. Hopefully you can get some better advice on this - I haven't kept Indian sticks for years, and am now keeping a smaller stick insect species so I can house more in the same place
Bear in mind that Indian sticks will all be female - which means once they're adult they will ALL start laying eggs which add up very quickly indeed! It is unwise to keep all the eggs as the hatch rate is very high and you could easily get overrun with hundreds of baby stick insects once they hatch

the eggs/ova take around 6 months to hatch.
Food - This is probably the main reason I asked what species you are getting. Most species will eat either bramble or privet leaves. Indian sticks happily eat privet, and I have heard that they can eat oak, ivy, rose and possibly a few others. They can also apparently eat bramble leaves too, but I have never tried this.
I would definitely recommend washing the leaves thoroughly before giving it to the sticks, though. Pesticides, pollution and other chemicals that can be on the leaves can kill sticks. I try to get my privet where there is little traffic or on very quiet roads just to minimise the risk.
Lastly, you will need to line the tank with something to catch the waste, and eventually, eggs. I use simple kitchen paper - its very easy to clean, and very easy to sort any eggs from the waste
It is unlikely you will need any heating pads - a warm room is just fine! The only thing you may need to go buy is a water spray bottle. All sticks appreciate a little bit of humidity (some more than others), and like to drink droplets from the leaves. You only need to lightly spray the leaves once a day - although as your tank has all solid walls you could probably get away with spraying the leaves once every other day. Be sure to not spray the stick insects, as they can drown, especially when young and small.
I hope this is of some help!
