Yes you can use it in any amount you want as many times
OK, assuming you have a BF account with your size bank the best way to do it is in smaller amounts on horses at $10+.
With the bonus you don't get your stake back if it wins so the shorter the odds the worse the return.
$500 @ $2 = $1000 less $500 stake so a return of $500 if it wins. Even if you could lay at say $2 you can only lay $250 to guarantee an equal return win or lose (less commission) so you'd be looking at a return of $230-$250 on your $500 bonus. Suboptimal.
Back a horse @ $20 for say $100. $2000 less $100 stake = $1900 collect if it wins. You can lay it at say $21 for say $90. $100 if it wins, $85 post comms if it loses. Rinse and repeat 4 more times. $425 profit from a $500 deposit bonus.
Longer odds = less impact on bonuses because the stake is not returned in the payout.
Because you can split the bet you should have enough to cover the lay side via BF