Number of keywords is not limited. You buy the service, or the time that someone spends working on your website and campaign. If you select one or a few keywords, it is not the same as ten. The time of the campaign is expressed in working hours, our advice is to choose the optimal number of keywords that can have a great effect on a specific period of optimization.
Sometimes it is better to focus on 2-3 keywords, rather than waste the "strength" on a large number of them and do not get the desired results.