How do I add more then two audio(tag) files in same Alexa intent function?

First of all total length cannot exceed 240s and there can be max 5 tags in the response: docs. To make sure that your audio works and fits the requirements, try pasting your audio tags in test console, 'Voice & Tone' tab. If it works there, then you are all set.

Now how to put them in one response, there are many options. You can use an array:

const audioUrls = [
  "<audio src='https:///blabla' />",
  "<audio src='https:///blabla' />",
]

return handlerInput.responseBuilder.speak(audioUrls.join(' '))

To not copy paste audio tags, you can build yourself a helper method, then code could look like this:

const audioTag = url => `<audio src=${url} />`
const audioUrls = [ 
  'https:///blabla', 
  'https:///blabla', 
] 
return handlerInput.responseBuilder
  .speak(audioUrls.map(url => audioTag(url)).join(' '))

If you want to do it the way you did, I would use template literals:

const audio1 = "<audio src='https:///blabla' />",
const audio2 = "<audio src='https:///blabla' />",
return handlerInput.responseBuilder.speak(`${audio1} ${audio2}`)