The 1,877 existing investors can only invest up to £1,000 each so they can only use up 1,877,000 of the 2.5m that’s been set aside for Crowdcube investors (& obviously some will invest less than £1,000) which means there will be a minimum of £623,000 to share out between new investors.
If the existing investors invest the average amount which was pledged on the first day of the latest campaign ~£283.50 (see this video at 8.05), then they would only take £532,129.5 leaving £1,967,870.50 for new investors…
If the new investors invest that much on average too, then there should be 6,941 new investors after this round is finished…
Using these figures, that means that if no one else pledged after today (current pledges are 8.95m), your odds of getting a chance to invest as a new investor are currently about 1 / 4.5 
Edit: My numbers are slightly off here, as there were actually 1,861 investors not 1,877, in the first round of fundraising - see Nathan’s post below.