Numel learns Flamethrower at level 41, which means its likely to stay a Numel before or after your battle with Tate & Liza, the 7th Gym Leader, assuming you don't go out of your way to farm EXP. I'd personally recommend you evolve Camerupt as soon as possible, and instead teach it a Fire type move through TM. Not only will Numel miss out on the stat increase it gains upon evolution for a sizeable duration of your playthrough, but also the use of any Fire type moves, solely relying on STAB Earthquake for damaging options, which is ineffective with a mere 60 base Attack stat. It may also be worth noting that Camerupt learns Eruption at level 45, which, while serving as a more unreliable, but far stronger alternative, is learnt just four levels after learning Flamethrower.
As for the Fire type move to use, Overheat is your best option. Not only is it easily accessible while progressing through the game, its got 140 total BP in Gen 3, and Camerupt isn't hindered by the Sp. Atk drop as much as other Pokemon would be, as the rest of its moveset is likely to be made up of non-Fire type Physical moves. Progressing through the game naturally should net you enough money to buy the Flamethrower TM, which you can use in place of or alongside Overheat to deal with Pokemon that don't require the additional BP. Fire Blast can serve as a cheaper and stronger, but less reliable alternative to Flamethrower, bought at the Lilycove Department Store.
A good moveset for Camerupt may look something like this:
Camerupt @ Soft Sand
Ability: Magma Armor
- Flamethrower / Fire Blast
- Earthquake
- Overheat / Eruption
- Return / Strength
