Just be patient! If you collect at a continuous rate, you will eventually get there. Buy what you like, and only buy expensive models if you are sure you want them. Good Luck!
How to expand a collection is a very personal subject, but ☝️ this is good advice.
There is absolutely nothing wrong with having a "small" collection as long as it is meaningful and rewarding to you. Nobody is going to give you a trophy based on the number of models you have. Don't just go buy a bunch of models just to have more models. Ideally, you'd want to get models that you can relate with at some level. The common themes are aircraft you have flown on, aircraft that you have seen or photographed, aircraft of historical relevance, aircraft that bring you good memories (nostalgia), etc...
We all like airplanes, so it is easy to get carried away with the monthly releases and what have you, but you'd have to do some soul-searching and find what is it that you truly like personally. I'm no financial advisor, but I'd say it is advisable not to buy models you cannot afford.