You can create a controller command that does that and call it in set intervals from a cronjob. The controller command will find the element that's storing your number, increment the number by the set interval and then save it. Something like this:
$entryWithNumberField = \craft\elements\Entry::find()
$currentNumber = $entryWithNumberField->getFieldValue('my_number_field');
$entryWithNumberField->setFieldValue('my_number_field', $currentNumber + 0.2);
However, this isn't a great solution as its very error prone. If the cron job fails for any reason (the server rebooting as part of a planned maintenance, for instance), the number will no longer be accurate.
Instead of saving the number every two hours, simply calculate the current number wherever you need to use it based on the defined starting point and the current time. This way, it will always be accurate, won't have the possibility for race conditions or be dependent on the previous state.
$entry = Entry::find()->my_number_field(':notempty:')->one();
$offset = $entry->my_number_field;
$now = time();
$start = $entry->dateCreated->getTimestamp();
$twoHourIntervalsPassed = floor($now - $start / 3600 / 2);
$incrementBy = $twoHourIntervalsPassed * 0.2;
$final = $offset + $incrementBy;