There’s no magic formula or even number but a couple of things that will really help is research and using more specific hashtags.
Spend a little time researching – here are a few things to try
Size matters – check how many times the hashtag has been used. If its really high your post will pretty much get lost. Try something a little more specific. Once you’ve found your smaller hashtag check the content just to make sure you and your brand will sit nicely there. Sometimes a hashtag can have a completely different meaning 
Avoid using the same hashtag’s in your post as the algorithm might think you’re a bot and your content is spammy (meaning the hashtag won’t work for you) Have a snoop at your competition to see what they’re using and also to see what their followers are using 
If you are looking for a local audience don’t forget to include area hashtag’sWhat hashtag tips do you have?
Need help? Get in touch I’d love to help 