How many natural numbers less than $1000$ can be formed from the digits $0,1,2,3,4,5$ when a digit may be repeated any number of times?

There are $5$ one-digit natural numbers out of the given digits.

For getting $2-digit$ natural numbers we cannot put $0$ at the ten's place. So, this place can be filled by any of the given five nonzero digits in $5$ ways.

The unit's digit can be filled by any of the given six digits in $6$ ways.

So, the number of $2-digit$ natural numbers $=(5\times 6)=30$.

Similarly, to get a $3-digit$ number, we cannot put on at the hundred's place. So, this place can be filled in $5$ ways, each of the ten's and unit's places can be filled in $6$ ways.

$\therefore$ the number of $3-digit$ numbers $=(5\times 6\times 6)=180$.

Required number of numbers $=(5+30+180)=215$.