This greatly depends on the way the core interaction of your marketplace takes place.
1) If in your marketplace the producers produce value (content, jobs, listings, riders etc, depending on what you market) and the consumers consume those listings, then you may want to have a plurality of producers because this will automatically unlock the other side of the marketplace, the consumer side.
Without enough producers you won't be able to attract consumers.
2) If in your marketplace producers respond to demand (for example bid for jobs/rides/products) the consumers have previously created, then having significantly more producers than consumers will create a problem for your marketplace.
One practice that can greatly assist you to balance supply and demand, if your problem is that one side of your marketplace is significantly bigger than the other,
is to encourage that one side to "play" the role of the other side.
For example, if you run a products' marketplace like Etsy, and your producers are many more than the consumers, what you can do is to try and spark those producers to also consume products from other craftsmen (producers). It's highly likely for a producer in a specific marketplace to end up being the consumer, too. It has happened to lots of marketplaces in different niches (transportation - i.e. Uber, accomodation - i.e. AirBnB, online laboring - i.e. oDesk) etc.
Lots of users who started on one side of the market ended up "playing" the role of the opposite side, in some cases to a big extent.
Other ways to "manually balance" the two sides of the marketplace, greatly depending on the stage and size your marketplace is on, is to manually try and solve the chicken and egg problem.
So, if your marketplace is for example offering Beaty Experts and you have more producers than consumers, what you do is you turn your focus on acquiring those consumers who match with your ample production.
This obviously depends highly on the domain/industry that you are in.
I can create a market place for, say, legal advice, tomorrow and have plenty of demand. I just need to make it a market place for free legal advice from top-lawyers. However, the supply will be hard to come by. Or I could get plenty of supply if I make it a marketplace for $1000/hr legal advice, no education required for the supplier. In which case the demand will be lacking. It can be quite tricky to find the equlibrium (indeed one might not exist) and I don't see how any generic advice could help. If you have managed this though, your problem should only be about reaching people. In that case, you either need to find the "home turf" of your suppliers or your buyers depending on which party you lack, go there and announce that people are looking to pay them money or people will help them solve their problem.