I have created a mobile app that requires I attract a lot of one type of user so that they can be reached by the other side in need of a service. Does anyone have tips of what percentage of that side I should work towards acquiring before I begin with the other side?
Your product will tell you. If you launch with 100 on the X side and 1 on the Y side and the Y bottlenecks due to volume, then you need more on the Y side. There is no formula, but most business statistics somehow magically fall into the 80/20 principle in more ways than one. At the start though, you may want to split your focus evenly until you have numbers that tell you which way put your focus. Starting a business like that is all about getting sample sizes and then scaling based on the results.
Perhaps instead of a % of your entire anticipated market prior to opening up to service-needers, think about grabing a much higher % of a smaller market. Open in one major city (as an example). When both sides are up move on to the next market area (which will be easier to gain users on both sides when they can see one market working already.
this always depends on the market how many users you need of one side to be relevant for the other.
But some things come to my mind to make it easier
1. which group is the decisive one to go for first and focus on that one. Take ebay: you won't get buyers without sellers but you can get sellers without buyers if you sell a good story to them where they can make revenue soon and as ebay did, for free.
2. how can you make the effort to get the initial group small. Think you you can narrow the first group down. For example if you link carpenters with end customers then start with your city or region first before expanding. Or if you connect experts with users who seek advice start with one focus area - say online marketing - first and expand later.